Modified transmission method for improving accuracy for E-911 calls

ABSTRACT

The accuracy of the location estimate of a Wireless Location System is dependent, in part, upon both the transmitted power of the wireless transmitter and the length in time of the transmission from the wireless transmitter. In general, higher power transmissions and transmissions of greater transmission length can be located with better accuracy by the Wireless Location System than lower power and shorter transmissions. Wireless communications systems generally limit the transmit power and transmission length of wireless transmitters in order to minimize interference within the communications system and to maximize the potential capacity of the system. The inventive method disclosed herein meets the conflicting needs of both systems by enabling the wireless communications system to minimize transmit power and length while enabling improved location accuracy for certain types of calls, such as wireless  9 - 1 - 1  calls. The method comprises the following steps: a wireless transmitter receives normal transmission parameters from a base station; the user of the wireless transmitter initiates a call on the wireless transmitter by dialing a sequence of digits and pressing “SEND” or “YES”; a processor within the wireless transmitter compares the dialed sequence of digits with one or more trigger events stored within the wireless transmitter; if the dialed sequence of digits does not match the trigger event, then the wireless transmitter uses the normal transmission parameters in making the call; and if the dialed sequence of digits matches the trigger event, then the wireless transmitter uses a modified transmission sequence.

[0001] CROSS REFERENCE TO RELATED APPLICATIONS

[0002] This is a continuation-in-part of U.S. patent application Ser.No. 09/227,764, filed on Jan. 8, 1999, entitled “Calibration forWireless Location System.”

FIELD OF THE INVENTION

[0003] The present invention relates generally to methods and apparatusfor locating wireless transmitters, such as those used in analog ordigital cellular systems, personal communications systems (PCS),enhanced specialized mobile radios (ESMRs), and other types of wirelesscommunications systems. This field is now generally known as wirelesslocation, and has application for Wireless E9-1-1, fleet management, RFoptimization, and other valuable applications.

BACKGROUND OF THE INVENTION

[0004] Early work relating to the present invention has been describedin U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone LocationSystem,” which discloses a system for locating cellular telephones usingnovel time difference of arrival (TDOA) techniques. Further enhancementsof the system disclosed in the '144 patent are disclosed in U.S. Pat.No. 5,608,410, Mar. 4, 1997, “System for Locating a Source of BurstyTransmissions.” Both patents are owned by the assignee of the currentinvention, and both are incorporated herein by reference. The presentinventors have continued to develop significant enhancements to theoriginal inventive concepts and have developed techniques to furtherimprove the accuracy of Wireless Location Systems while significantlyreducing the cost of these systems.

[0005] Over the past few years, the cellular industry has increased thenumber of air interface protocols available for use by wirelesstelephones, increased the number of frequency bands in which wireless ormobile telephones may operate, and expanded the number of terms thatrefer or relate to mobile telephones to include “personal communicationsservices”, “wireless”, and others. The air interface protocols nowinclude AMPS, N-AMPS, TDMA, CDMA, GSM,TACS, ESMR, GPRS, EDGE, andothers. The changes in terminology and increases in the number of airinterfaces do not change the basic principles and inventions discoveredand enhanced by the inventors. However, in keeping with the currentterminology of the industry, the inventors now call the system describedherein a Wireless Location System.

[0006] The inventors have conducted extensive experiments with theWireless Location System technology disclosed herein to demonstrate boththe viability and value of the technology. For example, severalexperiments were conducted during several months of 1995 and 1996 in thecities of Philadelphia and Baltimore to verify the system's ability tomitigate multipath in large urban environments. Then, in 1996 theinventors constructed a system in Houston that was used to test thetechnology's effectiveness in that area and its ability to interfacedirectly with E9-1-1 systems. Then, in 1997, the system was tested in a350 square mile area in New Jersey and was used to locate real 9-1-1calls from real people in trouble. Since that time, the system test hasbeen expanded to include 125 cell sites covering an area of over 2,000square miles. During all of these tests, techniques discussed anddisclosed herein were tested for effectiveness and further developed,and the system has been demonstrated to overcome the limitations ofother approaches that have been proposed for locating wirelesstelephones. Indeed, as of December, 1998, no other Wireless LocationSystem has been installed anywhere else in the world that is capable oflocating live 9-1-1 callers. The innovation of the Wireless LocationSystem disclosed herein has been acknowledged in the wireless industryby the extensive amount of media coverage given to the system'scapabilities, as well as by awards. For example, the prestigiousWireless Appy Award was granted to the system by the Cellular TelephoneIndustry Association in October, 1997, and the Christopher ColumbusFellowship Foundation and Discover Magazine found the Wireless LocationSystem to be one of the top 4 innovations of 1998 out of 4,000nominations submitted.

[0007] The value and importance of the Wireless Location System has beenacknowledged by the wireless communications industry. In June 1996, theFederal Communications Commission issued requirements for the wirelesscommunications industry to deploy location systems for use in locatingwireless 9-1-1 callers, with a deadline of October 2001. The location ofwireless E9-1-1 callers will save response time, save lives, and saveenormous costs because of reduced use of emergency responses resources.In addition, numerous surveys and studies have concluded that variouswireless applications, such as location sensitive billing, fleetmanagement, and others, will have great commercial values in the comingyears.

[0008] Background on Wireless Communications Systems

[0009] There are many different types of air interface protocols usedfor wireless communications systems. These protocols are used indifferent frequency bands, both in the U.S. and internationally. Thefrequency band does not impact the Wireless Location System'seffectiveness at locating wireless telephones.

[0010] All air interface protocols use two types of “channels”. Thefirst type includes control channels that are used for conveyinginformation about the wireless telephone or transmitter, for initiatingor terminating calls, or for transferring bursty data. For example, sometypes of short messaging services transfer data over the controlchannel. In different air interfaces, control channels are known bydifferent terminology, but the use of the control channels in each airinterface is similar. Control channels generally have identifyinginformation about the wireless telephone or transmitter contained in thetransmission. Control channels also include various data transferprotocols that are not voice specific—these include General Packet RadioService (GPRS), Enhanced Data rate for GSM Evolution (EDGE), andEnhanced GPRS (EGPRS).

[0011] The second type includes voice channels that are typically usedfor conveying voice communications over the air interface. Thesechannels are only used after a call has been set up using the controlchannels. Voice channels will typically use dedicated resources withinthe wireless communications system whereas control channels will useshared resources. This distinction will generally make the use ofcontrol channels for wireless location purposes more cost effective thanthe use of voice channels, although there are some applications forwhich regular location on the voice channel is desired. Voice channelsgenerally do not have identifying information about the wirelesstelephone or transmitter in the transmission. Some of the differences inthe air interface protocols are discussed below:

[0012] AMPS—This is the original air interface protocol used forcellular communications in the U.S. In the AMPS system, separatededicated channels are assigned for use by control channels (RCC).According to the TIA/EIA Standard IS-553A, every control channel blockmust begin at cellular channel 333 or 334, but the block may be ofvariable length. In the U.S., by convention, the AMPS control channelblock is 21 channels wide, but the use of a 26-channel block is alsoknown. A reverse voice channel (RVC) may occupy any channel that is notassigned to a control channel. The control channel modulation is FSK(frequency shift keying), while the voice channels are modulated usingFM (frequency modulation).

[0013] N-AMPS—This air interface is an expansion of the AMPS airinterface protocol, and is defined in EIA/TIA standard IS-88. Thecontrol channels are substantially the same as for AMPS, however, thevoice channels are different. The voice channels occupy less than 10 KHzof bandwidth, versus the 30 KHz used for AMPS, and the modulation is FM.

[0014] TDMA—This interface is also known D-AMPS, and is defined inEIA/TIA standard IS-136. This air interface is characterized by the useof both frequency and time separation. Control channels are known asDigital Control Channels (DCCH) and are transmitted in bursts intimeslots assigned for use by DCCH. Unlike AMPS, DCCH may be assignedanywhere in the frequency band, although there are generally somefrequency assignments that are more attractive than others based uponthe use of probability blocks. Voice channels are known as DigitalTraffic Channels (DTC). DCCH and DTC may occupy the same frequencyassignments, but not the same timeslot assignment in a given frequencyassignment. DCCH and DTC use the same modulation scheme, known as π/4DQPSK (differential quadrature phase shift keying). In the cellularband, a carrier may use both the AMPS and TDMA protocols, as long as thefrequency assignments for each protocol are kept separated. A carriermay also aggregate digital channels together to support higher speeddata transfer protocols such as GPRS and EDGE.

[0015] CDMA—This air interface is defined by EIA/TIA standard IS-95A.This air interface is characterized by the use of both frequency andcode separation. However, because adjacent cell sites may use the samefrequency sets, CDMA is also characterized by very careful powercontrol. This careful power control leads to a situation known to thoseskilled in the art as the near-far problem, which makes wirelesslocation difficult for most approaches to function properly. Controlchannels are known as Access Channels, and voice channels are known asTraffic Channels. Access and Traffic Channels may share the samefrequency band, but are separated by code. Access and Traffic Channelsuse the same modulation scheme, known as OQPSK. CDMA can support higherspeed data transfer protocols by aggregating codes together.

[0016] GSM—This air interface is defined by the international standardGlobal System for Mobile Communications. Like TDMA, GSM is characterizedby the use of both frequency and time separation. The channel bandwidthis 200 KHz, which is wider than the 30 KHz used for TDMA. Controlchannels are known as Standalone Dedicated Control Channels (SDCCH), andare transmitted in bursts in timeslots assigned for use by SDCCH. SDCCHmay be assigned anywhere in the frequency band. Voice channels are knownas Traffic Channels (TCH). SDCCH and TCH may occupy the same frequencyassignments, but not the same timeslot assignment in a given frequencyassignment. SDCCH and TCH use the same modulation scheme, known as GMSK.GSM can also support higher data transfer protocols such as GPRS andEGPRS.

[0017] Within this specification the reference to any one of the airinterfaces shall automatically refer to all of the air interfaces,unless specified otherwise. Additionally, a reference to controlchannels or voice channels shall refer to all types of control or voicechannels, whatever the preferred terminology for a particular airinterface. Finally, there are many more types of air interfaces usedthroughout the world, and there is no intent to exclude any airinterface from the inventive concepts described within thisspecification. Indeed, those skilled in the art will recognize otherinterfaces used elsewhere are derivatives of or similar in class tothose described above.

[0018] The preferred embodiments of the inventions disclosed herein havemany advantages over other techniques for locating wireless telephones.For example, some of these other techniques involve adding GPSfunctionality to telephones, which requires that significant changes bemade to the telephones. The preferred embodiments disclosed herein donot require such changes.

SUMMARY OF THE INVENTION

[0019] The accuracy of the location estimate of a Wireless LocationSystem is dependent, in part, upon both the transmitted power of thewireless transmitter and the length in time of the transmission from thewireless transmitter. In general, higher power transmissions andtransmissions of greater transmission length can be located with betteraccuracy by the Wireless Location System than lower power and shortertransmissions. Wireless communications systems generally limit thetransmit power and transmission length of wireless transmitters in orderto minimize interference within the communications system and tomaximize the potential capacity of the system.

[0020] An inventive method disclosed herein meets the conflicting needsof both systems by enabling the wireless communications system tominimize transmit power and length while enabling improved locationaccuracy for certain types of calls, such as wireless 9-1-1 calls. Thismethod comprises the following steps: a wireless transmitter receivesnormal transmission parameters from a base station; the user of thewireless transmitter initiates a call on the wireless transmitter bydialing a sequence of digits and pressing “SEND” or “YES”; a processorwithin the wireless transmitter compares the dialed sequence of digitswith one or more trigger events stored within the wireless transmitter;if the dialed sequence of digits does not match the trigger event, thenthe wireless transmitter uses the normal transmission parameters inmaking the call; and if the dialed sequence of digits matches thetrigger event, then the wireless transmitter uses a modifiedtransmission sequence.

[0021] Other features and advantages of the invention are disclosedbelow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIGS. 1 and 1A schematically depict a Wireless Location System inaccordance with the present invention.

[0023]FIG. 2 schematically depicts a Signal Collection System (SCS) 10in accordance with the present invention.

[0024]FIG. 2A schematically depicts a receiver module 10-2 employed bythe Signal Collection System.

[0025]FIGS. 2B and 2C schematically depict alternative ways of couplingthe receiver module(s) 10-2 to the antennas 10-1.

[0026]FIG. 2C-1 is a flowchart of a process employed by the WirelessLocation System when using narrowband receiver modules.

[0027]FIG. 2D schematically depicts a DSP module 10-3 employed in theSignal Collection System in accordance with the present invention.

[0028]FIG. 2E is a flowchart of the operation of the DSP module(s) 10-3,and FIG. 2E-1 is a flowchart of the process employed by the DSP modulesfor detecting active channels.

[0029]FIG. 2F schematically depicts a Control and Communications Module10-5 in accordance with the present invention.

[0030] FIGS. 2G-2J depict aspects of the presently preferred SCScalibration methods. FIG. 2G is a schematic illustration of baselinesand error values used to explain an external calibration method inaccordance with the present invention. FIG. 2H is a flowchart of aninternal calibration method. FIG. 2I is an exemplary transfer functionof an AMPS control channel and FIG. 2J depicts an exemplary comb signal.

[0031]FIGS. 2K and 2L are flowcharts of two methods for monitoringperformance of a Wireless Location System in accordance with the presentinvention.

[0032]FIG. 3 schematically depicts a TDOA Location Processor 12 inaccordance with the present invention.

[0033]FIG. 3A depicts the structure of an exemplary network mapmaintained by the TLP controllers in accordance with the presentinvention.

[0034]FIGS. 4 and 4A schematically depict different aspects of anApplications Processor 14 in accordance with the present invention.

[0035]FIG. 5 is a flowchart of a central station-based locationprocessing method in accordance with the present invention.

[0036]FIG. 6 is a flowchart of a station-based location processingmethod in accordance with the present invention.

[0037]FIG. 7 is a flowchart of a method for determining, for eachtransmission for which a location is desired, whether to employ centralor station-based processing.

[0038]FIG. 8 is a flowchart of a dynamic process used to selectcooperating antennas and SCS's 10 used in location processing.

[0039]FIG. 9 is diagram that is referred to below in explaining a methodfor selecting a candidate list of SCS's and antennas using apredetermined set of criteria.

[0040]FIGS. 10A and 10B are flowcharts of alternative methods forincreasing the bandwidth of a transmitted signal to improve locationaccuracy.

[0041] FIGS. 11A-11C are signal flow diagrams and FIG. 11D is aflowchart, and they are used to explain an inventive method forcombining multiple statistically independent location estimates toprovide an estimate with improved accuracy.

[0042]FIGS. 12A and 12B are a block diagram and a graph, respectively,for explaining a bandwidth synthesis method.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0043] The Wireless Location System (Wireless Location System) operatesas a passive overlay to a wireless communications system, such as acellular, PCS, or ESMR system, although the concepts are not limited tojust those types of communications systems. Wireless communicationssystems are generally not suitable for locating wireless devices becausethe designs of the wireless transmitters and cell sites do not includethe necessary functionality to achieve accurate location. Accuratelocation in this application is defined as accuracy of 100 to 400 feetRMS (root mean square). This is distinguished from the location accuracythat can be achieved by existing cell sites, which is generally limitedto the radius of the cell site. In general, cell sites are not designedor programmed to cooperate between and among themselves to determinewireless transmitter location. Additionally, wireless transmitters suchas cellular and PCS telephones are designed to be low cost and thereforegenerally do not have locating capability built-in. The WirelessLocation System is designed to be a low cost addition to a wirelesscommunications system that involves minimal changes to cell sites and nochanges at all to standard wireless transmitters. The Wireless LocationSystem is passive because it does not contain transmitters, andtherefore cannot cause interference of any kind to the wirelesscommunications system. The Wireless Location System uses only its ownspecialized receivers at cell sites or other receiving locations.

[0044] Overview of Wireless Location System (Wireless Location System)

[0045] As shown in FIG. 1, the Wireless Location System has four majorkinds of subsystems: the Signal Collection Systems (SCS's) 10, the TDOALocation Processors (TLP's) 12, the Application Processors (AP's) 14,and the Network Operations Console (NOC) 16. Each SCS is responsible forreceiving the RF signals transmitted by the wireless transmitters onboth control channels and voice channels. In general, each SCS ispreferably installed at a wireless carrier's cell site, and thereforeoperates in parallel to a base station. Each TLP 12 is responsible formanaging a network of SCS's 10 and for providing a centralized pool ofdigital signal processing (DSP) resources that can be used in thelocation calculations. The SCS's 10 and the TLP's 12 operate together todetermine the location of the wireless transmitters, as will bediscussed more fully below. Digital signal processing is the preferablemanner in which to process radio signals because DSP's are relativelylow cost, provide consistent performance, and are easily re-programmableto handle many different tasks. Both the SCS's 10 and TLP's 12 contain asignificant amount of DSP resources, and the software in these systemscan operate dynamically to determine where to perform a particularprocessing function based upon tradeoffs in processing time,communications time, queuing time, and cost. Each TLP 12 existscentrally primarily to reduce the overall cost of implementing theWireless Location System, although the techniques discussed herein arenot limited to the preferred architecture shown. That is, DSP resourcescan be relocated within the Wireless Location System without changingthe basic concepts and functionality disclosed.

[0046] The AP's 14 are responsible for managing all of the resources inthe Wireless Location System, including all of the SCS's 10 and TLP's12. Each AP 14 also contains a specialized database that contains“triggers” for the Wireless Location System. In order to conserveresources, the Wireless Location System can be programmed to locate onlycertain pre-determined types of transmissions. When a transmission of apre-determined type occurs, then the Wireless Location System istriggered to begin location processing. Otherwise, the Wireless LocationSystem may be programmed to ignore the transmission. Each AP 14 alsocontains applications interfaces that permit a variety of applicationsto securely access the Wireless Location System. These applications may,for example, access location records in real time or non-real time,create or delete certain type of triggers, or cause the WirelessLocation System to take other actions. Each AP 14 is also capable ofcertain post-processing functions that allow the AP 14 to combine anumber of location records to generate extended reports or analysesuseful for applications such as traffic monitoring or RF optimization.

[0047] The NOC 16 is a network management system that provides operatorsof the Wireless Location System easy access to the programmingparameters of the Wireless Location System. For example, in some cities,the Wireless Location System may contain many hundreds or even thousandsof SCS's 10. The NOC is the most effective way to manage a largeWireless Location System, using graphical user interface capabilities.The NOC will also receive real time alerts if certain functions withinthe Wireless Location System are not operating properly. These real timealerts can be used by the operator to take corrective action quickly andprevent a degradation of location service. Experience with trials of theWireless Location System show that the ability of the system to maintaingood location accuracy over time is directly related to the operator'sability to keep the system operating within its predeterminedparameters.

[0048] Readers of U.S. Pat. Nos. 5,327,144 and 5,608,410 and thisspecification will note similarities between the respective systems.Indeed, the system disclosed herein is significantly based upon and alsosignificantly enhanced from the system described in those previouspatents. For example, the SCS 10 has been expanded and enhanced from theAntenna Site System described in U.S. Pat. No. 5,608,410. The SCS 10 nowhas the capability to support many more antennas at a single cell site,and further can support the use of extended antennas as described below.This enables the SCS 10 to operate with the sectored cell sites nowcommonly used. The SCS 10 can also transfer data from multiple antennasat a cell site to the TLP 12 instead of always combining data frommultiple antennas before transfer. Additionally, the SCS 10 can supportmultiple air interface protocols thereby allowing the SCS 10 to functioneven as a wireless carrier continually changes the configuration of itssystem.

[0049] The TLP 12 is similar to the Central Site System disclosed inU.S. Pat. No. 5,608,410, but has also been expanded and enhanced. Forexample, the TLP 12 has been made scaleable so that the amount of DSPresources required by each TLP 12 can be appropriately scaled to matchthe number of locations per second required by customers of the WirelessLocation System. In order to support scaling for different WirelessLocation System capacities, a networking scheme has been added to theTLP 12 so that multiple TLP's 12 can cooperate to share RF data acrosswireless communication system network boundaries. Additionally, the TLP12 has been given control means to determine the SCS's 10, and moreimportantly the antennas at each of the SCS's 10, from which the TLP 12is to receive data in order to process a specific location. Previously,the Antenna Site Systems automatically forwarded data to the CentralSite System, whether requested or not by the Central Site System.Furthermore, the SCS 10 and TLP 12 combined have been designed withadditional means for removing multipath from the received transmissions.

[0050] The Database Subsystem of the Central Site System has beenexpanded and developed into the AP 14. The AP 14 can support a greatervariety of applications than previously disclosed in U.S. Pat. No.5,608,410, including the ability to post-process large volumes oflocation records from multiple wireless transmitters. Thispost-processed data can yield, for example, very effective maps for useby wireless carriers to improve and optimize the RF design of thecommunications systems. This can be achieved, for example, by plottingthe locations of all of the callers in an area and the received signalstrengths at a number of cell sites. The carrier can then determinewhether each cell site is, in fact, serving the exact coverage areadesired by the carrier. The AP 14 can also now store location recordsanonymously, that is, with the MIN and/or other identity informationremoved from the location record, so that the location record can beused for RF optimization or traffic monitoring without causing concernsabout an individual user's privacy.

[0051] As shown in FIG. 1A, a presently preferred implementation of theWireless Location System includes a plurality of SCS regions each ofwhich comprises multiple SCS's 10. For example, “SCS Region 1” includesSCS's 10A and 10B (and preferably others, not shown) that are located atrespective cell sites and share antennas with the base stations at thosecell sites. Drop and insert units 11A and 11B are used to interfacefractional T1/E1 lines to full T1/E1 lines, which in turn are coupled toa digital access and control system (DACS) 13A. The DACS 13A and anotherDACS 13B are used in the manner described more fully below forcommunications between the SCS's 10A, 10B, etc., and multiple TLP's 12A,12B, etc. As shown, the TLP's are typically collocated andinterconnected via an Ethernet network (backbone) and a second,redundant Ethernet network. Also coupled to the Ethernet networks aremultiple AP's 14A and 14B, multiple NOC's 16A and 16B, and a terminalserver 15. Routers 19A and 19B are used to couple one Wireless LocationSystem to one or more other Wireless Location System(s).

[0052] Signal Collection System 10

[0053] Generally, cell sites will have one of the following antennaconfigurations: (i) an omnidirectional site with 1 or 2 receive antennasor (ii) a sectored site with 1, 2, or 3 sectors, and with 1 or 2 receiveantennas used in each sector. As the number of cell sites has increasedin the U.S. and internationally, sectored cell sites have become thepredominant configuration. However, there are also a growing number ofmicro-cells and pico-cells, which can be omnidirectional. Therefore, theSCS 10 has been designed to be configurable for any of these typicalcell sites and has been provided with mechanisms to employ any number ofantennas at a cell site.

[0054] The basic architectural elements of the SCS 10 remain the same asfor the Antenna Site System described in U.S. Pat. No. 5,608,410, butseveral enhancements have been made to increase the flexibility of theSCS 10 and to reduce the commercial deployment cost of the system. Themost presently preferred embodiment of the SCS 10 is described herein.The SCS 10, an overview of which is shown in FIG. 2, includes digitalreceiver modules 10-2A through 10-2C; DSP modules 10-3A through 10-3C; aserial bus 10-4, a control and communications module 10-5; a GPS module10-6; and a clock distribution module 10-7. The SCS 10 has the followingexternal connections: power, fractional T1/E1 communications, RFconnections to antennas, and a GPS antenna connection for the timinggeneration (or clock distribution) module 10-7. The architecture andpackaging of the SCS 10 permit it to be physically collocated with cellsites (which is the most common installation place), located at othertypes of towers (such as FM, AM, two-way emergency communications,television, etc.), or located at other building structures (such asrooftops, silos, etc.).

[0055] Timing Generation

[0056] The Wireless Location System depends upon the accuratedetermination of time at all SCS's 10 contained within a network.Several different timing generation systems have been described inprevious disclosures, however the most presently preferred embodiment isbased upon an enhanced GPS receiver 10-6. The enhanced GPS receiverdiffers from most traditional GPS receivers in that the receivercontains algorithms that remove some of the timing instability of theGPS signals, and guarantees that any two SCS's 10 contained within anetwork can receive timing pulses that are within approximately tennanoseconds of each other. These enhanced GPS receivers are nowcommercially available, and further reduce some of the time referencerelated errors that were observed in previous implementations ofwireless location systems. While this enhanced GPS receiver can producea very accurate time reference, the output of the receiver may stillhave an unacceptable phase noise. Therefore, the output of the receiveris input to a low phase noise, crystal oscillator-driven phase lockedloop circuit that can now produce 10 MHz and one pulse per second (PPS)reference signals with less than 0.01 degrees RMS of phase noise, andwith the pulse output at any SCS 10 in a Wireless Location Systemnetwork within ten nanoseconds of any other pulse at another SCS 10.This combination of enhanced GPS receiver, crystal oscillator, and phaselocked loop is now the most preferred method to produce stable time andfrequency reference signals with low phase noise.

[0057] The SCS 10 has been designed to support multiple frequency bandsand multiple carriers with equipment located at the same cell site. Thiscan take place by using multiple receivers internal to a single SCSchassis, or by using multiple chassis each with separate receivers. Inthe event that multiple SCS chassis are placed at the same cell site,the SCS's 10 can share a single timing generation/clock distributioncircuit 10-7 and thereby reduce overall system cost. The 10 MHz and onePPS output signals from the timing generation circuit are amplified andbuffered internal to the SCS 10, and then made available via externalconnectors. Therefore a second SCS can receive its timing from a firstSCS using the buffered output and the external connectors. These signalscan also be made available to base station equipment collocated at thecell site. This might be useful to the base station, for example, inimproving the frequency re-use pattern of a wireless communicationssystem.

[0058] Receiver Module 10-2 (Wideband Embodiment)

[0059] When a wireless transmitter makes a transmission, the WirelessLocation System must receive the transmission at multiple SCS's 10located at multiple geographically dispersed cell sites. Therefore, eachSCS 10 has the ability to receive a transmission on any RF channel onwhich the transmission may originate. Additionally, since the SCS 10 iscapable of supporting multiple air interface protocols, the SCS 10 alsosupports multiple types of RF channels. This is in contrast to mostcurrent base station receivers, which typically receive only one type ofchannel and are usually capable of receiving only on select RF channelsat each cell site. For example, a typical TDMA base station receiverwill only support 30 KHz wide channels, and each receiver is programmedto receive signals on only a single channel whose frequency does notchange often (i.e. there is a relatively fixed frequency plan).Therefore, very few TDMA base station receivers would receive atransmission on any given frequency. As another example, even thoughsome GSM base station receivers are capable of frequency hopping, thereceivers at multiple base stations are generally not capable ofsimultaneously tuning to a single frequency for the purpose ofperforming location processing. In fact, the receivers at GSM basestations are programmed to frequency hop to avoid using an RF channelthat is being used by another transmitter so as to minimizeinterference.

[0060] The SCS receiver module 10-2 is preferably a dual widebanddigital receiver that can receive the entire frequency band and all ofthe RF channels of an air interface. For cellular systems in the U.S.,this receiver module is either 15 MHz wide or 25 MHz wide so that all ofthe channels of a single carrier or all of the channels of both carrierscan be received. This receiver module has many of the characteristics ofthe receiver previously described in U.S. Pat. No. 5,608,410, and FIG.2A is a block diagram of the currently preferred embodiment. Eachreceiver module contains an RF tuner section 10-2-1, a data interfaceand control section 10-2-2 and an analog to digital conversion section10-2-3. The RF tuner section 10-2-1 includes two full independentdigital receivers (including Tuner #1 and Tuner #2) that convert theanalog RF input from an external connector into a digitized data stream.Unlike most base station receivers, the SCS receiver module does notperform diversity combining or switching. Rather, the digitized signalfrom each independent receiver is made available to the locationprocessing. The present inventors have determined that there is anadvantage to the location processing, and especially the multipathmitigation processing, to independently process the signals from eachantenna rather than perform combining on the receiver module.

[0061] The receiver module 10-2 performs, or is coupled to elements thatperform, the following functions: automatic gain control (to supportboth nearby strong signals and far away weak signals), bandpassfiltering to remove potentially interfering signals from outside of theRF band of interest, synthesis of frequencies needed for mixing with theRF signals to create an IF signal that can be sampled, mixing, andanalog to digital conversion (ADC) for sampling the RF signals andoutputting a digitized data stream having an appropriate bandwidth andbit resolution. The frequency synthesizer locks the synthesizedfrequencies to the 10 MHz reference signal from the clockdistribution/timing generation module 10-7 (FIG. 2). All of the circuitsused in the receiver module maintain the low phase noise characteristicsof the timing reference signal. The receiver module preferably has aspurious free dynamic range of at least 80 dB.

[0062] The receiver module 10-2 also contains circuits to generate testfrequencies and calibration signals, as well as test ports wheremeasurements can be made by technicians during installation ortroubleshooting. Various calibration processes are described in furtherdetail below. The internally generated test frequencies and test portsprovide an easy method for engineers and technicians to rapidly test thereceiver module and diagnose any suspected problems. This is alsoespecially useful during the manufacturing process.

[0063] One of the advantages of the Wireless Location System describedherein is that no new antennas are required at cell sites. The WirelessLocation System can use the existing antennas already installed at mostcell sites, including both omni-directional and sectored antennas. Thisfeature can result in significant savings in the installation andmaintenance costs of the Wireless Location System versus otherapproaches that have been described in the prior art. The SCS's digitalreceivers 10-2 can be connected to the existing antennas in two ways, asshown in FIGS. 2B and 2C, respectively. In FIG. 2B, the SCS receivers10-2 are connected to the existing cell site multi-coupler or RFsplitter. In this manner, the SCS 10 uses the cell site's existing lownoise pre-amplifier, band pass filter, and multi-coupler or RF splitter.This type of connection usually limits the SCS 10 to supporting thefrequency band of a single carrier. For example, an A-side cellularcarrier will typically use the band pass filter to block signals fromcustomers of the B-side carrier, and vice versa.

[0064] In FIG. 2C, the existing RF path at the cell site has beeninterrupted, and a new pre-amplifier, band pass filter, and RF splitterhas been added as part of the Wireless Location System. The new bandpass filter will pass multiple contiguous frequency bands, such as boththe A-side and B-side cellular carriers, thereby allowing the WirelessLocation System to locate wireless transmitters using both cellularsystems but using the antennas from a single cell site. In thisconfiguration, the Wireless Location System uses matched RF componentsat each cell site, so that the phase versus frequency responses areidentical. This is in contrast to existing RF components, which may befrom different manufacturers or using different model numbers at variouscell sites. Matching the response characteristics of RF componentsreduces a possible source of error for the location processing, althoughthe Wireless Location System has the capability to compensate for thesesources of error. Finally, the new pre-amplifier installed with theWireless Location System will have a very low noise figure to improvethe sensitivity of the SCS 10 at a cell site. The overall noise figureof the SCS digital receivers 10-2 is dominated by the noise figure ofthe low noise amplifiers. Because the Wireless Location System can useweak signals in location processing, whereas the base station typicallycannot process weak signals, the Wireless Location System cansignificantly benefit from a high quality, very low noise amplifier.

[0065] In order to improve the ability of the Wireless Location Systemto accurately determine TDOA for a wireless transmission, the phaseversus frequency response of the cell site's RF components aredetermined at the time of installation and updated at other certaintimes and then stored in a table in the Wireless Location System. Thiscan be important because, for example, the band pass filters and/ormulti-couplers made by some manufacturers have a steep and non-linearphase versus frequency response near the edge of the pass band. If theedge of the pass band is very near to or coincident with the reversecontrol or voice channels, then the Wireless Location System would makeincorrect measurements of the transmitted signal's phase characteristicsif the Wireless Location System did not correct the measurements usingthe stored characteristics. This becomes even more important if acarrier has installed multi-couplers and/or band pass filters from morethan one manufacturer, because the characteristics at each site may bedifferent. in addition to measuring the phase versus frequency response,other environmental factors may cause changes to the RF path prior tothe ADC. These factors require occasional and sometimes periodiccalibration in the SCS 10.

[0066] Alternative Narrowband Embodiment of Receiver Module 10-2

[0067] In addition or as an alternative to the wideband receiver module,the SCS 10 also supports a narrowband embodiment of the receiver module10-2. In contrast to the wideband receiver module that cansimultaneously receive all of the RF channels in use by a wirelesscommunications system, the narrowband receiver can only receive one or afew RF channels at a time. For example, the SCS 10 supports a 60 KHznarrowband receiver for use in AMPS/TDMA systems, covering twocontiguous 30 KHz channels. This receiver is still a digital receiver asdescribed for the wideband module, however the frequency synthesizingand mixing circuits are used to dynamically tune the receiver module tovarious RF channels on command. This dynamic tuning can typically occurin one millisecond or less, and the receiver can dwell on a specific RFchannel for as long as required to receive and digitize RF data forlocation processing.

[0068] The purpose of the narrowband receiver is to reduce theimplementation cost of a Wireless Location System from the cost that isincurred with wideband receivers. Of course, there is some loss ofperformance, but the availability of these multiple receivers permitswireless carriers to have more cost/performance options. Additionalinventive functions and enhancements have been added to the WirelessLocation System to support this new type of narrowband receiver. Whenthe wideband receiver is being used, all RF channels are receivedcontinuously at all SCS's 10, and subsequent to the transmission, theWireless Location System can use the DSP's 10-3 (FIG. 2) to dynamicallyselect any RF channel from the digital memory. With the narrowbandreceiver, the Wireless Location System must ensure a priori that thenarrowband receivers at multiple cell sites are simultaneously tuned tothe same RF channel so that all receivers can simultaneously receive,digitize and store the same wireless transmission. For this reason, thenarrowband receiver is generally used only for locating voice channeltransmissions, which can be known a priori to be making a transmission.Since control channel transmissions can occur asynchronously at anytime, the narrowband receiver may not be tuned to the correct channel toreceive the transmission.

[0069] When the narrowband receivers are used for locating AMPS voicechannel transmissions, the Wireless Location System has the ability totemporarily change the modulation characteristics of the AMPS wirelesstransmitter to aid location processing. This may be necessary becauseAMPS voice channels are only FM modulated with the addition of a lowlevel supervisory tone known as SAT. As is known in the art, theCramer-Rao lower bound of AMPS FM modulation is significantly worse thanthe Manchester encoded FSK modulation used for AMPS reverse channels and“blank and burst” transmissions on the voice channel. Further, AMPSwireless transmitters may be transmitting with significantly reducedenergy if there is no modulating input signal (i.e., no one isspeaking). To improve the location estimate by improving the modulationcharacteristics without depending on the existence or amplitude of aninput modulating signal, the Wireless Location System can cause an AMPSwireless transmitter to transmit a “blank and burst” message at a pointin time when the narrowband receivers at multiple SCS's 10 are tuned tothe RF channel on which the message will be sent. This is furtherdescribed later.

[0070] The Wireless Location System performs the following steps whenusing the narrowband receiver module (see the flowchart of FIG. 2C-1):

[0071] a first wireless transmitter is a priori engaged in transmittingon a particular RF channel;

[0072] the Wireless Location System triggers to make a location estimateof the first wireless transmitter (the trigger may occur eitherinternally or externally via a command/response interface);

[0073] the Wireless Location System determines the cell site, sector, RFchannel, timeslot, long code mask, and encryption key (all informationelements may not be necessary for all air interface protocols) currentlyin use by the first wireless transmitter;

[0074] the Wireless Location System tunes an appropriate firstnarrowband receiver at an appropriate first SCS 10 to the RF channel andtimeslot at the designated cell site and sector, wherein appropriatetypically means both available and collocated or in closest proximity;

[0075] the first SCS 10 receives a time segment of RF data, typicallyranging from a few microseconds to tens of milliseconds, from the firstnarrowband receiver and evaluates the transmission's power, SNR, andmodulation characteristics;

[0076] if the transmission's power or SNR is below a predeterminedthreshold, the Wireless Location System waits a predetermined length oftime and then returns to the above third step (where the WirelessLocation System determines the cell site, sector, etc.);

[0077] if the transmission is an AMPS voice channel transmission and themodulation is below a threshold, then the Wireless Location Systemcommands the wireless communications system to send a command to thefirst wireless transmitter to cause a “blank and burst” on the firstwireless transmitter;

[0078] the Wireless Location System requests the wireless communicationssystem to prevent hand-off of the wireless transmitter to another RFchannel for a predetermined length of time;

[0079] the Wireless Location System receives a response from thewireless communications system indicating the time period during whichthe first wireless transmitter will be prevented from handing-off, andif commanded, the time period during which the wireless communicationssystem will send a command to the first wireless transmitter to cause a“blank and burst”;

[0080] the Wireless Location System determines the list of antennas thatwill be used in location processing (the antenna selection process isdescribed below);

[0081] the Wireless Location System determines the earliest WirelessLocation System timestamp at which the narrowband receivers connected tothe selected antennas are available to begin simultaneously collectingRF data from the RF channel currently in use by the first wirelesstransmitter;

[0082] based upon the earliest Wireless Location System timestamp andthe time periods in the response from the wireless communicationssystem, the Wireless Location System commands the narrowband receiversconnected to the antennas that will be used in location processing totune to the cell site, sector, and RF channel currently in use by thefirst wireless transmitter and to receive RF data for a predetermineddwell time (based upon the bandwidth of the signal, SNR, and integrationrequirements);

[0083] the RF data received by the narrowband receivers are written intothe dual port memory;

[0084] location processing on the received RF data commences, asdescribed in U.S. Pat. Nos. 5,327,144 and 5,608,410 and in sectionsbelow;

[0085] the Wireless Location System again determines the cell site,sector, RF channel, timeslot, long code mask, and encryption keycurrently in use by the first wireless transmitter;

[0086] if the cell site, sector, RF channel, timeslot, long code mask,and encryption key currently in use by the first wireless transmitterhas changed between queries (i.e. before and after gathering the RFdata) the Wireless Location System ceases location processing, causes analert message that location processing failed because the wirelesstransmitter changed transmission status during the period of time inwhich RF data was being received, and re-triggers this entire process;

[0087] location processing on the received RF data completes inaccordance with the steps described below.

[0088] The determination of the information elements including cellsite, sector, RF channel, timeslot, long code mask, and encryption key(all information elements may not be necessary for all air interfaceprotocols) is typically obtained by the Wireless Location System througha command/response interface between the Wireless Location System andthe wireless communications system.

[0089] The use of the narrowband receiver in the manner described aboveis known as random tuning because the receivers can be directed to anyRF channel on command from the system. One advantage to random tuning isthat locations are processed only for those wireless transmitters forwhich the Wireless Location System is triggered. One disadvantage torandom tuning is that various synchronization factors, including theinterface between the wireless communications system and the WirelessLocation System and the latency times in scheduling the necessaryreceivers throughout the system, can limit the total location processingthroughput. For example, in a TDMA system, random tuning used throughoutthe Wireless Location System will typically limit location processingthroughput to about 2.5 locations per second per cell site sector.

[0090] Therefore, the narrowband receiver also supports another mode,known as automatic sequential tuning, which can perform locationprocessing at a higher throughput. For example, in a TDMA system, usingsimilar assumptions about dwell time and setup time as for thenarrowband receiver operation described above, sequential tuning canachieve a location processing throughput of about 41 locations persecond per cell site sector, meaning that all 395 TDMA RF channels canbe processed in about 9 seconds. This increased rate can be achieved bytaking advantage of, for example, the two contiguous RF channels thatcan be received simultaneously, location processing all three TDMAtimeslots in an RF channel, and eliminating the need for synchronizationwith the wireless communications system. When the Wireless LocationSystem is using the narrowband receivers for sequential tuning, theWireless Location System has no knowledge of the identity of thewireless transmitter because the Wireless Location System does not waitfor a trigger, nor does the Wireless Location System query the wirelesscommunications system for the identity information prior to receivingthe transmission. In this method, the Wireless Location System sequencesthrough every cell site, RF channel and time slot, performs locationprocessing, and reports a location record identifying a time stamp, cellsite, RF channel, time slot, and location. Subsequent to the locationrecord report, the Wireless Location System and the wirelesscommunications system match the location records to the wirelesscommunications system's data indicating which wireless transmitters werein use at the time, and which cell sites, RF channels, and time slotswere used by each wireless transmitter. Then, the Wireless LocationSystem can retain the location records for wireless transmitters ofinterest, and discard those location records for the remaining wirelesstransmitters.

[0091] Digital Signal Processor Module 10-3

[0092] The SCS digital receiver modules 10-2 output a digitized RF datastream having a specified bandwidth and bit resolution. For example, a15 MHz embodiment of the wideband receiver may output a data streamcontaining 60 million samples per second, at a resolution of 14 bits persample. This RF data stream will contain all of the RF channels that areused by the wireless communications system. The DSP modules 10-3 receivethe digitized data stream, and can extract any individual RF channelthrough digital mixing and filtering. The DSP's can also reduce the bitresolution upon command from the Wireless Location System, as needed toreduce the bandwidth requirements between the SCS 10 and TLP 12. TheWireless Location System can dynamically select the bit resolution atwhich to forward digitized baseband RF data, based upon the processingrequirements for each location. DSP's are used for these functions toreduce the systemic errors that can occur from mixing and filtering withanalog components. The use of DSP's allows perfect matching in theprocessing between any two SCS's 10.

[0093] A block diagram of the DSP module 10-3 is shown is FIG. 2D, andthe operation of the DSP module is depicted by the flowchart of FIG. 2E.As shown in FIG. 2D, the DSP module 10-3 comprises the followingelements: a pair of DSP elements 10-3-1A and 10-3-1B, referred tocollectively as a “first” DSP; serial to parallel converters 10-3-2;dual port memory elements 10-3-3; a second DSP 10-3-4; a parallel toserial converter; a FIFO buffer; a DSP 10-3-5 (including RAM) fordetection, another DSP 10-3-6 for demodulation, and another DSP 10-3-7for normalization and control; and an address generator 10-3-8. In apresently preferred embodiment, the DSP module 10-3 receives thewideband digitized data stream (FIG. 2E, step S1), and uses the firstDSP (10-3-1A and 10-3-1B) to extract blocks of channels (step S2). Forexample, a first DSP programmed to operate as a digital drop receivercan extract four blocks of channels, wherein each block includes atleast 1.25 MHz of bandwidth. This bandwidth can include 42 channels ofAMPS or TDMA, 6 channels of GSM, or 1 channel of CDMA. The DSP does notrequire the blocks to be contiguous, as the DSP can independentlydigitally tune to any set of RF channels within the bandwidth of thewideband digitized data stream. The DSP can also perform wideband ornarrow band energy detection on all or any of the channels in the block,and report the power levels by channel to the TLP 12 (step S3). Forexample, every 10 ms, the DSP can perform wideband energy detection andcreate an RF spectral map for all channels for all receivers (see stepS9). Because this spectral map can be sent from the SCS 10 to the TLP 12every 10 ms via the communications link connecting the SCS 10 and theTLP 12, a significant data overhead could exist. Therefore, the DSPreduces the data overhead by companding the data into a finite number oflevels. Normally, for example, 84 dB of dynamic range could require 14bits. In the companding process implemented by the DSP, the data isreduced, for example, to only 4 bits by selecting 16 important RFspectral levels to send to the TLP 12. The choice of the number oflevels, and therefore the number of bits, as well as the representationof the levels, can be automatically adjusted by the Wireless LocationSystem. These adjustments are performed to maximize the informationvalue of the RF spectral messages sent to the TLP 12 as well as tooptimize the use of the bandwidth available on the communications linkbetween the SCS 10 and the TLP 12.

[0094] After conversion, each block of RF channels (each at least 1.25MHz) is passed through serial to parallel converter 10-3-2 and thenstored in dual port digital memory 10-3-3 (step S4). The digital memoryis a circular memory, which means that the DSP module begins writingdata into the first memory address and then continues sequentially untilthe last memory address is reached. When the last memory address isreached, the DSP returns to the first memory address and continues tosequentially write data into memory. Each DSP module typically containsenough memory to store several seconds of data for each block of RFchannels to support the latency and queuing times in the locationprocess.

[0095] In the DSP module, the memory address at which digitized andconverted RF data is written into memory is the time stamp usedthroughout the Wireless Location System and which the locationprocessing references in determining TDOA. In order to ensure that thetime stamps are aligned at every SCS 10 in the Wireless Location System,the address generator 10-3-8 receives the one pulse per second signalfrom the timing generation/clock distribution module 10-7 (FIG. 2).Periodically, the address generator at all SCS's 10 in a WirelessLocation System will simultaneously reset themselves to a known address.This enables the location processing to reduce or eliminate accumulatedtiming errors in the recording of time stamps for each digitized dataelement.

[0096] The address generator 10-3-8 controls both writing to and readingfrom the dual port digital memory 10-3-3. Writing takes placescontinuously since the ADC is continuously sampling and digitizing RFsignals and the first DSP (10-3-1A and 10-3-1B) is continuouslyperforming the digital drop receiver function. However, reading occursin bursts as the Wireless Location System requests data for performingdemodulation and location processing. The Wireless Location System mayeven perform location processing recursively on a single transmission,and therefore requires access to the same data multiple times. In orderto service the many requirements of the Wireless Location System, theaddress generator allows the dual port digital memory to be read at arate faster than the writing occurs. Typically, reading can be performedeight times faster than writing.

[0097] The DSP module 10-3 uses the second DSP 10-3-4 to read the datafrom the digital memory 10-3-3, and then performs a second digital dropreceiver function to extract baseband data from the blocks of RFchannels (step S5). For example, the second DSP can extract any single30 KHz AMPS or TDMA channel from any block of RF channels that have beendigitized and stored in the memory. Likewise, the second DSP can extractany single GSM channel. The second DSP is not required to extract a CDMAchannel, since the channel bandwidth occupies the full bandwidth of thestored RF data. The combination of the first DSP 10-3-1A, 10-3-1B andthe second DSP 10-3-4 allows the DSP module to select, store, andrecover any single RF channel in a wireless communications system. A DSPmodule typically will store four blocks of channels. In a dual-modeAMPS/TDMA system, a single DSP module can continuously andsimultaneously monitor up to 42 analog reverse control channels, up to84 digital control channels, and also be tasked to monitor and locateany voice channel transmission. A single SCS chassis will typicallysupport up to three receiver modules 10-2 (FIG. 2), to cover threesectors of two antennas each, and up to nine DSP modules (three DSPmodules per receiver permits an entire 15 MHz bandwidth to besimultaneously stored into digital memory). Thus, the SCS 10 is a verymodular system than can be easily scaled to match any type of cell siteconfiguration and processing load.

[0098] The DSP module 10-3 also performs other functions, includingautomatic detection of active channels used in each sector (step S6),demodulation (step S7), and station based location processing (step S8).The Wireless Location System maintains an active map of the usage of theRF channels in a wireless communications system (step S9), which enablesthe Wireless Location System to manage receiver and processingresources, and to rapidly initiate processing when a particulartransmission of interest has occurred. The active map comprises a tablemaintained within the Wireless Location System that lists for eachantenna connected to an SCS 10 the primary channels assigned to that SCS10 and the protocols used in those channels. A primary channel is an RFcontrol channel assigned to a collocated or nearby base station whichthe base station uses for communications with wireless transmitters. Forexample, in a typical cellular system with sectored cell sites, therewill be one RF control channel frequency assigned for use in eachsector. Those control channel frequencies would typically be assigned asprimary channels for a collocated SCS 10.

[0099] The same SCS 10 may also be assigned to monitor the RF controlchannels of other nearby base stations as primary channels, even ifother SCS's 10 also have the same primary channels assigned. In thismanner, the Wireless Location System implements a system demodulationredundancy that ensures that any given wireless transmission has aninfinitesimal probability of being missed. When this demodulationredundancy feature is used, the Wireless Location System will receive,detect, and demodulate the same wireless transmission two or more timesat more than one SCS 10. The Wireless Location System includes means todetect when this multiple demodulation has occurred and to triggerlocation processing only once. This function conserves the processingand communications resources of the Wireless Location System, and isfurther described below. This ability for a single SCS 10 to detect anddemodulate wireless transmissions occurring at cell sites not collocatedwith the SCS 10 permits operators of the Wireless Location System todeploy more efficient Wireless Location System networks. For example,the Wireless Location System may be designed such that the WirelessLocation System uses much fewer SCS's 10 than the wirelesscommunications system has base stations.

[0100] In the Wireless Location System, primary channels are entered andmaintained in the table using two methods: direct programming andautomatic detection. Direct programming comprises entering primarychannel data into the table using one of the Wireless Location Systemuser interfaces, such as the Network Operations Console 16 (FIG. 1), orby receiving channel assignment data from the Wireless Location Systemto wireless communications system interface. Alternatively, the DSPmodule 10-3 also runs a background process known as automatic detectionin which the DSP uses spare or scheduled processing capacity to detecttransmissions on various possible RF channels and then attempt todemodulate those transmissions using probable protocols. The DSP modulecan then confirm that the primary channels directly programmed arecorrect, and can also quickly detect changes made to channels at basestation and send an alert to the operator of the Wireless LocationSystem.

[0101] The DSP module performs the following steps in automaticdetection (see FIG. 2E-1):

[0102] for each possible control and/or voice channel which may be usedin the coverage area of the SCS 10, peg counters are established (stepS7-1);

[0103] at the start of a detection period, all peg counters are reset tozero (step S7-2);

[0104] each time that a transmission occurs in a specified RF channel,and the received power level is above a particular pre-set threshold,the peg counter for that channel is incremented (step S7-3);

[0105] each time that a transmission occurs in a specified RF channel,and the received power level is above a second particular pre-setthreshold, the DSP module attempts to demodulate a certain portion ofthe transmission using a first preferred protocol (step S7-4);

[0106] if the demodulation is successful, a second peg counter for thatchannel is incremented (step S7-5);

[0107] if the demodulation is unsuccessful, the DSP module attempts todemodulate a portion of the transmission using a second preferredprotocol (step S7-6);

[0108] if the demodulation is successful, a third peg counter for thatchannel is incremented (step S7-7);

[0109] at the end of a detection period, the Wireless Location Systemreads all peg counters (step S7-8); and

[0110] the Wireless Location System automatically assigns primarychannels based upon the peg counters (step S7-9).

[0111] The operator of the Wireless Location System can review the pegcounters and the automatic assignment of primary channels anddemodulation protocols, and override any settings that were performedautomatically. In addition, if more than two preferred protocols may beused by the wireless carrier, then the DSP module 10-3 can be downloadedwith software to detect the additional protocols. The architecture ofthe SCS 10, based upon wideband receivers 10-2, DSP modules 10-3, anddownloadable software permits the Wireless Location System to supportmultiple demodulation protocols in a single system. There is asignificant cost advantage to supporting multiple protocols within thesingle system, as only a single SCS 10 is required at a cell site. Thisis in contrast to many base station architectures, which may requiredifferent transceiver modules for different modulation protocols. Forexample, while the SCS 10 could support AMPS, TDMA, and CDMAsimultaneously in the same SCS 10, there is no base station currentlyavailable that can support this functionality.

[0112] The ability to detect and demodulate multiple protocols alsoincludes the ability to independently detect the use of authenticationin messages transmitted over the certain air interface protocols. Theuse of authentication fields in wireless transmitters started to becomeprevalent within the last few years as a means to reduce the occurrenceof fraud in wireless communications systems. However, not all wirelesstransmitters have implemented authentication. When authentication isused, the protocol generally inserts an additional field into thetransmitted message. Frequently this field is inserted between theidentity of the wireless transmitter and the dialed digits in thetransmitted message. When demodulating a wireless transmission, theWireless Location System determines the number of fields in thetransmitted message, as well as the message type (i.e. registration,origination, page response, etc.). The Wireless Location Systemdemodulates all fields and if extra fields appear to be present, givingconsideration to the type of message transmitted, then the WirelessLocation System tests all fields for a trigger condition. For example,if the dialed digits “911” appear in the proper place in a field, andthe field is located either in its proper place without authenticationor its proper place with authentication, then the Wireless LocationSystem triggers normally. In this example, the digits “911” would berequired to appear in sequence as “911” or “*911”, with no other digitsbefore or after either sequence. This functionality reduces oreliminates a false trigger caused by the digits “911” appearing as partof an authentication field.

[0113] The support for multiple demodulation protocols is important forthe Wireless Location System to successfully operate because locationprocessing must be quickly triggered when a wireless caller has dialed“911”. The Wireless Location System can trigger location processingusing two methods: the Wireless Location System will independentlydemodulate control channel transmissions, and trigger locationprocessing using any number of criteria such as dialed digits, or theWireless Location System may receive triggers from an external sourcesuch as the carrier's wireless communications system. The presentinventors have found that independent demodulation by the SCS 10 resultsin the fastest time to trigger, as measured from the moment that awireless user presses the “SEND” or “TALK” (or similar) button on awireless transmitter.

[0114] Control and Communications Module 10-5

[0115] The control and communications module 10-5, depicted in FIG. 2F,includes data buffers 10-5-1, a controller 10-5-2, memory 10-5-3, a CPU10-5-4 and a T1/E1 communications chip 10-5-5. The module has many ofthe characteristics previously described in U.S. Pat. No. 5,608,410.Several enhancements have been added in the present embodiment. Forexample, the SCS 10 now includes an automatic remote reset capability,even if the CPU on the control and communications module ceases toexecute its programmed software. This capability can reduce theoperating costs of the Wireless Location System because technicians arenot required to travel to a cell site to reset an SCS 10 if it fails tooperate normally. The automatic remote reset circuit operates bymonitoring the communications interface between the SCS 10 and the TLP12 for a particular sequence of bits. This sequence of bits is asequence that does not occur during normal communications between theSCS 10 and the TLP 12. This sequence, for example, may consist of an allones pattern. The reset circuit operates independently of the CPU sothat even if the CPU has placed itself in a locked or othernon-operating status, the circuit can still achieve the reset of the SCS10 and return the CPU to an operating status.

[0116] This module now also has the ability to record and report a widevariety of statistics and variables used in monitoring or diagnosing theperformance of the SCS 10. For example, the SCS 10 can monitor thepercent capacity usage of any DSP or other processor in the SCS 10, aswell as the communications interface between the SCS 10 and the TLP 12.These values are reported regularly to the AP 14 and the NOC 16, and areused to determine when additional processing and communicationsresources are required in the system. For example, alarm thresholds maybe set in the NOC to indicate to an operator if any resource isconsistently exceeding a preset threshold. The SCS 10 can also monitorthe number of times that transmissions have been successfullydemodulated, as well as the number of failures. This is useful inallowing operators to determine whether the signal thresholds fordemodulation have been set optimally.

[0117] This module, as well as the other modules, can also self-reportits identity to the TLP 12. As described below, many SCS's 10 can beconnected to a single TLP 12. Typically, the communications betweenSCS's 10 and TLP's 12 is shared with the communications between basestations and MSC's. It is frequently difficult to quickly determineexactly which SCS's 10 have been assigned to particular circuits.Therefore, the SCS 10 contains a hard coded identity, which is recordedat the time of installation. This identity can be read and verified bythe TLP 12 to positively determine which SCS 10 has been assigned by acarrier to each of several different communications circuits.

[0118] The SCS to TLP communications supports a variety of messages,including: commands and responses, software download, status andheartbeat, parameter download, diagnostic, spectral data, phase data,primary channel demodulation, and RF data. The communications protocolis designed to optimize Wireless Location System operation by minimizingthe protocol overhead and the protocol includes a message priorityscheme. Each message type is assigned a priority, and the SCS 10 and theTLP 12 will queue messages by priority such that a higher prioritymessage is sent before a lower priority message is sent. For example,demodulation messages are generally set at a high priority because theWireless Location System must trigger location processing on certaintypes of calls (i.e., E9-1-1) without delay. Although higher prioritymessages are queued before lower priority messages, the protocolgenerally does not preempt a message that is already in transit. Thatis, a message in the process of being sent across the SCS 10 to TLP 12communications interface will be completed fully, but then the nextmessage to be sent will be the highest priority message with theearliest time stamp. In order to minimize the latency of high prioritymessages, long messages, such as RF data, are sent in segments.

[0119] For example, the RF data for a full 100-millisecond AMPStransmission may be separated into 10-millisecond segments. In thismanner, a high priority message may be queued in between segments of theRF data.

[0120] Calibration and Performance Monitoring

[0121] The architecture of the SCS 10 is heavily based upon digitaltechnologies including the digital receiver and the digital signalprocessors. Once RF signals have been digitized, timing, frequency, andphase differences can be carefully controlled in the various processes.More importantly, any timing, frequency, and phase differences can beperfectly matched between the various receivers and various SCS's 10used in the Wireless Location System. However, prior to the ADC, the RFsignals pass through a number of RF components, including antennas,cables, low noise amplifiers, filters, duplexors, multi-couplers, and RFsplitters. Each of these RF components has characteristics important tothe Wireless Location System, including delay and phase versus frequencyresponse. When the RF and analog components are perfectly matchedbetween the pairs of SCS's 10, such as SCS 10A and SCS 10B in FIG. 2G,then the effects of these characteristics are automatically eliminatedin the location processing. But when the characteristics of thecomponents are not matched, then the location processing caninadvertently include instrumental errors resulting from the mismatch.Additionally, many of these RF components can experience instabilitywith power, time, temperature, or other factors that can addinstrumental errors to the determination of location. Therefore, severalinventive techniques have been developed to calibrate the RF componentsin the Wireless Location System and to monitor the performance of theWireless Location System on a regular basis. Subsequent to calibration,the Wireless Location System stores the values of these delays andphases versus frequency response (i.e. by RF channel number) in a tablein the Wireless Location System for use in correcting these instrumentalerrors. FIGS. 2G-2J are referred to below in explaining thesecalibration methods.

[0122] External Calibration Method

[0123] Referring to FIG. 2G, the timing stability of the WirelessLocation System is measured along baselines, wherein each baseline iscomprised of two SCS's, 10A and 10B, and an imaginary line (A-B) drawnbetween them. In a TDOA/FDOA type of Wireless Location System, locationsof wireless transmitters are calculated by measuring the differences inthe times that each SCS 10 records for the arrival of the signal from awireless transmitter. Thus, it is important that the differences intimes measured by SCS's 10 along any baseline are largely attributed tothe transmission time of the signal from the wireless transmitter andminimally attributed to the variations in the RF and analog componentsof the SCS's 10 themselves. To meet the accuracy goals of the WirelessLocation System, the timing stability for any pair of SCS's 10 aremaintained at much less than 100 nanoseconds RMS (root mean square).Thus, the components of the Wireless Location System will contributeless than 100 feet RMS of instrumentation error in the estimation of thelocation of a wireless transmitter. Some of this error is allocated tothe ambiguity of the signal used to calibrate the system. This ambiguitycan be determined from the well-known Cramer-Rao lower bound equation.In the case of an AMPS reverse control channel, this error isapproximately 40 nanoseconds RMS. The remainder of the error budget isallocated to the components of the Wireless Location System, primarilythe RF and analog components in the SCS 10.

[0124] In the external calibration method, the Wireless Location Systemuses a network of calibration transmitters whose signal characteristicsmatch those of the target wireless transmitters. These calibrationtransmitters may be ordinary wireless telephones emitting periodicregistration signals and/or page response signals. Each usableSCS-to-SCS baseline is preferably calibrated periodically using acalibration transmitter that has a relatively clear and unobstructedpath to both SCS's 10 associated with the baseline. The calibrationsignal is processed identically to a signal from a target wirelesstransmitter. Since the TDOA values are known a priori, any errors in thecalculations are due to systemic errors in the Wireless Location System.These systemic errors can then be removed in the subsequent locationcalculations for target transmitters.

[0125]FIG. 2G illustrates the external calibration method for minimizingtiming errors. As shown, a first SCS 10A at a point “A” and a second SCS10A at a point “B” have an associated baseline A-B. A calibration signalemitted at time T₀ by a calibration transmitter at point “C” willtheoretically reach first SCS 10A at time T₀+T_(AC). T_(AC) is a measureof the amount of time required for the calibration signal to travel fromthe antenna on the calibration transmitter to the dual port digitalmemory in a digital receiver. Likewise, the same calibration signal willreach second SCS 10B at a theoretical time T₀+T_(BC). Usually, however,the calibration signal will not reach the digital memory and the digitalsignal processing components of the respective SCS's 10 at exactly thecorrect times. Rather, there will be errors e1 and e2 in the amount oftime (T_(AC), T_(BC)) it takes the calibration signal to propagate fromthe calibration transmitter to the SCS's 10, respectively, such that theexact times of arrival are actually T₀+T_(AC)+e1 and T₀+T_(BC)+e2. Sucherrors will be due to some extent to delays in the signal propagationthrough the air, i.e., from the calibration transmitter's antenna to theSCS antennas; however, the errors will be due primarily to time varyingcharacteristics in the SCS front end components. The errors e1 and e2cannot be determined per se because the system does not know the exacttime (T₀) at which the calibration signal was transmitted. The systemcan, however, determine the error in the difference in the time ofarrival of the calibration signal at the respective SCS's 10 of anygiven pair of SCS's 10. This TDOA error value is defined as thedifference between the measured TDOA value and the theoretical TDOAvalue τ₀, wherein τ₀ is the theoretical differences between thetheoretical delay values T_(AC) and T_(BC). Theoretical TDOA values foreach pair of SCS's 10 and each calibration transmitter are known becausethe positions of the SCS's 10 and calibration transmitter, and the speedat which the calibration signal propagates, are known. The measured TDOAbaseline (TDOA_(A-B)) can be represented as TDOA_(A-B=τ) ₀+ε, whereinε=e1−e2. In a similar manner, a calibration signal from a secondcalibration transmitter at point “D” will have associated errors e3 ande4. The ultimate value of ε to be subtracted from TDOA measurements fora target transmitter will be a function (e.g., weighted average) of theε values derived for one or more calibration transmitters. Therefore, agiven TDOA measurement (TDOA_(measured)) for a pair of SCS's 10 atpoints “X” and “Y” and a target wireless transmitter at an unknownlocation will be corrected as follows: TDOA_(x-y) = TDOA_(measured) − εε = k1ε1 + k2ε2 + . . . kNεN,

[0126] where k1, k2, etc., are weighting factors and ε1, ε2, etc., arethe errors determined by subtracting the measured TDOA values from thetheoretical values for each calibration transmitter. In this example,error value ε1 may the error value associated with the calibrationtransmitter at point “C” in the drawing. The weighting factors aredetermined by the operator of the Wireless Location System, and inputinto the configuration tables for each baseline. The operator will takeinto consideration the distance from each calibration transmitter to theSCS's 10 at points “X” and “Y”, the empirically determined line of sightfrom each calibration transmitter to the SCS's 10 at points “X” and “Y”,and the contribution that each SCS “X” and “Y” would have made to alocation estimate of a wireless transmitter that might be located in thevicinity of each calibration transmitter. In general, calibrationtransmitters that are nearer to the SCS's 10 at points “X” and “Y” willbe weighted higher than calibration transmitters that are farther away,and calibration transmitters with better line of sight to the SCS's 10at points “X” and “Y” will be weighted higher than calibrationtransmitters with worse line of sight.

[0127] Each error component e1, e2, etc., and therefore the resultingerror component ε, can vary widely, and wildly, over time because someof the error component is due to multipath reflection from thecalibration transmitter to each SCS 10. The multipath reflection is verymuch path dependent and therefore will vary from measurement tomeasurement and from path to path. It is not an object of this method todetermine the multipath reflection for these calibration paths, butrather to determine the portion of the errors that are attributable tothe components of the SCS's 10. Typically, therefore, error values e1and e3 will have a common component since they relate to the same firstSCS 10A. Likewise, error values e2 and e4 will also have a commoncomponent since they relate to the second SCS 10B. It is known thatwhile the multipath components can vary wildly, the component errorsvary slowly and typically vary sinusoidally. Therefore, in the externalcalibration method, the error values ε are filtered using a weighted,time-based filter that decreases the weight of the wildly varyingmultipath components while preserving the relatively slow changing errorcomponents attributed to the SCS's 10. One such exemplary filter used inthe external calibration method is the Kalman filter.

[0128] The period between calibration transmissions is varied dependingon the error drift rates determined for the SCS components. The periodof the drift rate should be much longer than the period of thecalibration interval. The Wireless Location System monitors the periodof the drift rate to determine continuously the rate of change, and mayperiodically adjust the calibration interval, if needed. Typically, thecalibration rate for a Wireless Location System such as one inaccordance with the present invention is between 10 and 30 minutes. Thiscorresponds well with the typical time period for the registration ratein a wireless communications system. If the Wireless Location Systemwere to determine that the calibration interval must be adjusted to arate faster than the registration rate of the wireless communicationssystem, then the AP 14 (FIG. 1) would automatically force thecalibration transmitter to transmit by paging the transmitter at theprescribed interval. Each calibration transmitter is individuallyaddressable and therefore the calibration interval associated with eachcalibration transmitter can be different.

[0129] Since the calibration transmitters used in the externalcalibration method are standard telephones, the Wireless Location Systemmust have a mechanism to distinguish those telephones from the otherwireless transmitters that are being located for various applicationpurposes. The Wireless Location System maintains a list of theidentities of the calibration transmitters, typically in the TLP 12 andin the AP 14. In a cellular system, the identity of the calibrationtransmitter can be the Mobile Identity Number, or MIN. When thecalibration transmitter makes a transmission, the transmission isreceived by each SCS 10 and demodulated by the appropriate SCS 10. TheWireless Location System compares the identity of the transmission witha pre-stored tasking list of identities of all calibration transmitters.If the Wireless Location System determines that the transmission was acalibration transmission, then the Wireless Location System initiatesexternal calibration processing.

[0130] Internal Calibration Method

[0131] In addition to the external calibration method, it is an objectof the present invention to calibrate all channels of the widebanddigital receiver used in the SCS 10 of a Wireless Location System. Theexternal calibration method will typically calibrate only a singlechannel of the multiple channels used by the wideband digital receiver.This is because the fixed calibration transmitters will typically scanto the highest-power control channel, which will typically be the samecontrol channel each time. The transfer function of a wideband digitalreceiver, along with the other associated components, does not remainperfectly constant, however, and will vary with time and temperature.Therefore, even though the external calibration method can successfullycalibrate a single channel, there is no assurance that the remainingchannels will also be calibrated.

[0132] The internal calibration method, represented in the flowchart ofFIG. 2H, is particularly suited for calibrating an individual firstreceiver system (i.e., SCS 10) that is characterized by a time- andfrequency-varying transfer function, wherein the transfer functiondefines how the amplitude and phase of a received signal will be alteredby the receiver system and the receiver system is utilized in a locationsystem to determine the location of a wireless transmitter by, in part,determining a difference in time of arrival of a signal transmitted bythe wireless transmitter and received by the receiver system to becalibrated and another receiver system, and wherein the accuracy of thelocation estimate is dependent, in part, upon the accuracy of TDOAmeasurements made by the system. An example of a AMPS RCC transferfunction is depicted in FIG. 2I, which depicts how the phase of thetransfer function varies across the 21 control channels spanning 630KHz.

[0133] Referring to FIG. 2H, the internal calibration method includesthe steps of temporarily and electronically disconnecting the antennaused by a receiver system from the receiver system (step S-20);injecting an internally generated wideband signal with known and stablesignal characteristics into the first receiver system (step S-21);utilizing the generated wideband signal to obtain an estimate of themanner in which the transfer function varies across the bandwidth of thefirst receiver system (step S-22); and utilizing the estimate tomitigate the effects of the variation of the first transfer function onthe time and frequency measurements made by the first receiver system(step S-23). One example of a stable wideband signal used for internalcalibration is a comb signal, which is comprised of multiple individual,equal-amplitude frequency elements at a known spacing, such as 5 KHz. Anexample of such a signal is shown in FIG. 2I.

[0134] The antenna must be temporarily disconnected during the internalcalibration process to prevent external signals from entering thewideband receiver and to guarantee that the receiver is only receivingthe stable wideband signal. The antenna is electronically disconnectedonly for a few milliseconds to minimize the chance of missing too muchof a signal from a wireless transmitter. In addition, internalcalibration is typically performed immediately after externalcalibration to minimize the possibility that the any component in theSCS 10 drifts during the interval between external and internalcalibration. The antenna is disconnected from the wideband receiverusing two electronically controlled RF relays (not shown). An RF relaycannot provide perfect isolation between input and output even when inthe “off” position, but it can provide up to 70 dB of isolation. Tworelays may be used in series to increase the amount of isolation and tofurther assure that no signal is leaked from the antenna to the widebandreceiver during calibration. Similarly, when the internal calibrationfunction is not being used, the internal calibration signal is turnedoff, and the two RF relays are also turned off to prevent leakage of theinternal calibration signals into the wideband receiver when thereceiver is collecting signals from wireless transmitters.

[0135] The external calibration method provides an absolute calibrationof a single channel and the internal calibration method then calibrateseach other channel relative to the channel that had been absolutelycalibrated. The comb signal is particularly suited as a stable widebandsignal because it can be easily generated using a stored replica of thesignal and a digital to analog converter.

[0136] External Calibration Using Wideband Calibration Signal

[0137] The external calibration method described next may be used inconnection with an SCS 10 receiver system characterized by a time- andfrequency-varying transfer function, which preferably includes theantennas, filters, amplifiers, duplexors, multi-couplers, splitters, andcabling associated with the SCS receiver system. The method includes thestep of transmitting a stable, known wideband calibration signal from anexternal transmitter. The wideband calibration signal is then used toestimate the transfer function across a prescribed bandwidth of the SCSreceiver system. The estimate of the transfer function is subsequentlyemployed to mitigate the effects of variation of the transfer functionon subsequent TDOA/FDOA measurements. The external transmission ispreferably of short duration and low power to avoid interference withthe wireless communications system hosting the Wireless Location System.

[0138] In the preferred method, the SCS receiver system is synchronizedwith the external transmitter. Such synchronization may be performedusing GPS timing units. Moreover, the receiver system may be programmedto receive and process the entire wideband of the calibration signalonly at the time that the calibration signal is being sent. The receiversystem will not perform calibration processing at any time other thanwhen in synchronization with the external calibration transmissions. Inaddition, a wireless communications link is used between the receiversystem and the external calibration transmitter to exchange commands andresponses. The external transmitter may use a directional antenna todirect the wideband signal only at the antennas of the SCS receiversystem. Such as directional antenna may be a Yagi antenna (i.e. linearend-fire array). The calibration method preferably includes making theexternal transmission only when the directional antenna is aimed at thereceiver system's antennas and the risk of multipath reflection is low.

[0139] Calibrating for Station Biases

[0140] Another aspect of the present invention concerns a calibrationmethod to correct for station biases in a SCS receiver system. The“station bias” is defined as the finite delay between when an RF signalfrom a wireless transmitter reaches the antenna and when that samesignal reached the wideband receiver. The inventive method includes thestep of measuring the length of the cable from the antennas to thefilters and determining the corresponding delays associated with thecable length. In addition, the method includes injecting a known signalinto the filter, duplexor, multi-coupler, or RF splitter and measuringthe delay and phase response versus frequency response from the input ofeach device to the wideband receiver. The delay and phase values arethen combined and used to correct subsequent location measurements. Whenused with the GPS based timing generation described above, the methodpreferably includes correcting for the GPS cable lengths. Moreover, anexternally generated reference signal is preferably used to monitorchanges in station bias that may arise due to aging and weather.Finally, the station bias by RF channel and for each receiver system inthe Wireless Location System is preferably stored in tabular form in theWireless Location System for use in correcting subsequent locationprocessing.

[0141] Performance Monitoring

[0142] The Wireless Location System uses methods similar to calibrationfor performance monitoring on a regular and ongoing basis. These methodsare depicted in the flowcharts of FIGS. 2K and 2L. Two methods ofperformance monitoring are used: fixed phones and drive testing ofsurveyed points. The fixed phone method comprises the following steps(see FIG. 2K):

[0143] standard wireless transmitters are permanently placed at variouspoints within the coverage area of the Wireless Location System (theseare then known as the fixed phones) (step S-30);

[0144] the points at which the fixed phones have been placed aresurveyed so that their location is precisely known to within apredetermined distance, for example ten feet (step S-31);

[0145] the surveyed locations are stored in a table in the AP 14 (stepS-32);

[0146] the fixed phones are permitted to register on the wirelesscommunications system, at the rate and interval set by the wirelesscommunications system for all wireless transmitters on the system (stepS-33);

[0147] at each registration transmission by a fixed phone, the WirelessLocation System locates the fixed phone using normal location processing(as with the calibration transmitters, the Wireless Location System canidentify a transmission as being from a fixed phone by storing theidentities in a table) (step S-34);

[0148] the Wireless Location System computes an error between thecalculated location determined by the location processing and the storedlocation determined by survey (step S-35);

[0149] the location, the error value, and other measured parameters arestored along with a time stamp in a database in the AP 14 (step S-36);

[0150] the AP 14 monitors the instant error and other measuredparameters (collectively referred to as an extended location record) andadditionally computes various statistical values of the error(s) andother measured parameters (step S-37); and

[0151] if any of the error or other values exceed a pre-determinedthreshold or a historical statistical value, either instantaneously orafter performing statistical filtering over a prescribed number oflocation estimates, the AP 14 signals an alarm to the operator of theWireless Location System (step S-38).

[0152] The extended location record includes a large number of measuredparameters usefully for analyzing the instant and historical performanceof the Wireless Location System. These parameters include: the RFchannel used by the wireless transmitter, the antenna port(s) used bythe Wireless Location System to demodulate the wireless transmission,the antenna ports from which the Wireless Location System requested RFdata, the peak, average, and variance in power of the transmission overthe interval used for location processing, the SCS 10 and antenna portchosen as the reference for location processing, the correlation valuefrom the cross-spectra correlation between every other SCS 10 andantenna used in location processing and the reference SCS 10 andantenna, the delay value for each baseline, the multipath mitigationparameters, and the residual values remaining after the multipathmitigation calculations. Any of these measured parameters can bemonitored by the Wireless Location System for the purpose of determininghow the Wireless Location System is performing. One example of the typeof monitoring performed by the Wireless Location System may be thevariance between the instant value of the correlation on a baseline andthe historical range of the correlation value. Another may be thevariance between the instant value of the received power at a particularantenna and the historical range of the received power. Many otherstatistical values can be calculated and this list is not exhaustive.

[0153] The number of fixed phones placed into the coverage area of theWireless Location System can be determined based upon the density of thecell sites, the difficulty of the terrain, and the historical ease withwhich wireless communications systems have performed in the area.Typically the ratio is about one fixed phone for every six cell sites,however in some areas a ratio of one to one may be required. The fixedphones provide a continuous means to monitor the performance of theWireless Location System, as well as the monitor any changes in thefrequency plan that the carrier may have made. Many times, changes inthe frequency plan will cause a variation in the performance of theWireless Location System and the performance monitoring of the fixedphones provide an immediate indication to the Wireless Location Systemoperator.

[0154] Drive testing of surveyed points is very similar to the fixedphone monitoring. Fixed phones typically can only be located indoorswhere access to power is available (i.e. the phones must be continuouslypowered on to be effective). To obtain a more complete measurement ofthe performance of the location performance, drive testing of outdoortest points is also performed. Referring to FIG. 2L, as with the fixedphones, prescribed test points throughout the coverage area of theWireless Location System are surveyed to within ten feet (step S-40).Each test point is assigned a code, wherein the code consists of eithera “*” or a “#”, followed by a sequence number (step S-41). For example,“*1001” through “*1099” may be a sequence of 99 codes used for testpoints. These codes should be sequences, that when dialed, aremeaningless to the wireless communications system (i.e. the codes do notcause a feature or other translation to occur in the MSC, except for anintercept message). The AP 14 stores the code for each test point alongwith the surveyed location (step S-42). Subsequent to these initialsteps, any wireless transmitter dialing any of the codes will betriggered and located using normal location processing (steps S-43 andS-44). The Wireless Location System automatically computes an errorbetween the calculated location determined by the location processingand the stored location determined by survey, and the location and theerror value are stored along with a time stamp in a database in the AP14 (steps S-45 and S-46). The AP 14 monitors the instant error, as wellas various historical statistical values of the error. If the errorvalues exceed a pre-determined threshold or a historical statisticalvalue, either instantaneously or after performing statistical filteringover a prescribed number of location estimates, the AP 14 signals analarm to the operator of the Wireless Location System (step S-47).

[0155] TDOA Location Processor (TLP)

[0156] The TLP 12, depicted in FIGS. 1, 1A and 3, is a centralizeddigital signal processing system that manages many aspects of theWireless Location System, especially the SCS's 10, and provides controlover the location processing. Because location processing is DSPintensive, one of the major advantages of the TLP 12 is that the DSPresources can be shared among location processing initiated bytransmissions at any of the SCS's 10 in a Wireless Location System. Thatis, the additional cost of DSP's at the SCS's 10 is reduced by havingthe resource centrally available. As shown in FIG. 3, there are threemajor components of the TLP 12: DSP modules 12-1, T1/E1 communicationsmodules 12-2 and a controller module 12-3.

[0157] The T1/E1 communications modules 12-2 provide the communicationsinterface to the SCS's 10 (T1 and E1 are standard communications speedsavailable throughout the world). Each SCS 10 communicates to a TLP 12using one or more DSO's (which are typically 56 Kbps or 64 Kbps). EachSCS 10 typically connects to a fractional T1 or E1 circuit, using, e.g.,a drop and insert unit or channel bank at the cell site. Frequently,this circuit is shared with the base station, which communicates withthe MSC. At a central site, the DSO's assigned to the base station areseparated from the DSO's assigned to the SCS's 10. This is typicallyaccomplished external to the TLP 12 using a digital access and controlsystem (DACS) 13A that not only separates the DSO's but also grooms theDSO's from multiple SCS's 10 onto full T1 or E1 circuits. These circuitsthen connect from the DACS 13A to the DACS 13B and then to the T1/E1communications module on the TLP 12. Each T1/E1 communications modulecontains sufficient digital memory to buffer packets of data to and fromeach SCS 10 communicating with the module. A single TLP chassis maysupport one or more T1/E1 communications modules.

[0158] The DSP modules 12-1 provide a pooled resource for locationprocessing. A single module may typically contain two to eight digitalsignal processors, each of which are equally available for locationprocessing. Two types of location processing are supported: centralbased and station based, which are described in further detail below.The TLP controller 12-3 manages the DSP module(s) 12-1 to obtain optimalthroughput. Each DSP module contains sufficient digital memory to storeall of the data necessary for location processing. A DSP is not engageduntil all of the data necessary to begin location processing has beenmoved from each of the involved SCS's 10 to the digital memory on theDSP module. Only then is a DSP given the specific task to locate aspecific wireless transmitter. Using this technique, the DSP's, whichare an expensive resource, are never kept waiting. A single TLP chassismay support one or more DSP modules.

[0159] The controller module 12-3 provides the real time management ofall location processing within the Wireless Location System. The AP 14is the top-level management entity within the Wireless Location System,however its database architecture is not sufficiently fast to conductthe real time decision making when transmissions occur. The controllermodule 12-3 receives messages from the SCS's 10, including: status,spectral energy in various channels for various antennas, demodulatedmessages, and diagnostics. This enables the controller to continuouslydetermine events occurring in the Wireless Location System, as well asto send commands to take certain actions. When a controller modulereceives demodulated messages from SCS's 10, the controller moduledecides whether location processing is required for a particularwireless transmission. The controller module 12-3 also determines whichSCS's 10 and antennas to use in location processing, including whetherto use central based or station based location processing. Thecontroller module commands SCS's 10 to return the necessary data, andcommands the communications modules and DSP modules to sequentiallyperform their necessary roles in location processing. These steps aredescribed below in further detail.

[0160] The controller module 12-3 maintains a table known as the Signalof Interest Table (SOIT). This table contains all of the criteria thatmay be used to trigger location processing on a particular wirelesstransmission. The criteria may include, for example, the Mobile IdentityNumber, the Mobile Station ID, the Electronic Serial Number, dialeddigits, System ID, RF channel number, cell site number or sector number,type of transmission, and other types of data elements. Some of thetrigger events may have higher or lower priority levels associated withthem for use in determining the order of processing. Higher prioritylocation triggers will always be processing before lower prioritylocation triggers. However, a lower priority trigger that has alreadybegun location processing will complete the processing before beingassigned to a higher priority task. The master Tasking List for theWireless Location System is maintained on the AP 14, and copies of theTasking List are automatically downloaded to the Signal of InterestTable in each TLP 12 in the Wireless Location System. The full Signal ofInterest Table is downloaded to a TLP 12 when the TLP 12 is reset orfirst starts. Subsequent to those two events, only changes aredownloaded from the AP 14 to each TLP 12 to conserve communicationsbandwidth. The TLP 12 to AP 14 communications protocol preferablycontains sufficient redundancy and error checking to prevent incorrectdata from ever being entered into the Signal of Interest Table. When theAP 14 and TLP 12 periodically have spare processing capacity available,the AP 14 reconfirms entries in the Signal of Interest Table to ensurethat all Signal of Interest Table entries in the Wireless LocationSystem are in full synchronization.

[0161] Each TLP chassis has a maximum capacity associated with thechassis. For example, a single TLP chassis may only have sufficientcapacity to support between 48 and 60 SCS's 10. When a wirelesscommunications system is larger that the capacity of a single TLPchassis, multiple TLP chassis are connected together using Ethernetnetworking. The controller module 12-3 is responsible for inter-TLPcommunications and networking, and communicates with the controllermodules in other TLP chassis and with Application Processors 14 over theEthernet network. Inter-TLP communications is required when locationprocessing requires the use of SCS's 10 that are connected to differentTLP chassis. Location processing for each wireless transmission isassigned to a single DSP module in a single TLP chassis. The controllermodules 12-3 in TLP chassis select the DSP module on which to performlocation processing, and then route all of the RF data used in locationprocessing to that DSP module. If RF data is required from the SCS's 10connected to more that one TLP 12, then the controller modules in allnecessary TLP chassis communicate to move the RF data from all necessarySCS's 10 to their respective connected TLP's 12 and then to the DSPmodule and TLP chassis assigned to the location processing. Thecontroller module supports two fully independent Ethernet networks forredundancy. A break or failure in any one network causes the affectedTLP's 12 to immediately shift all communications to the other network.

[0162] The controller modules 12-3 maintain a complete network map ofthe Wireless Location System, including the SCS's 10 associated witheach TLP chassis. The network map is a table stored in the controllermodule containing a list of the candidate SCS/antennas that may be usedin location processing, and various parameters associated with each ofthe SCS/antennas. The structure of an exemplary network map is depictedin FIG. 3A. There is a separate entry in the table for each antennaconnected to an SCS 10. When a wireless transmission occurs in an areathat is covered by SCS's 10 communicating with more than one TLPchassis, the controller modules in the involved TLP chassis determinewhich TLP chassis will be the “master” TLP chassis for the purpose ofmanaging location processing. Typically, the TLP chassis associated withthe SCS 10 that has the primary channel assignment for the wirelesstransmission is assigned to be the master. However, another TLP chassismay be assigned instead if that TLP temporarily has no DSP resourcesavailable for location processing, or if most of the SCS's 10 involvedin location processing are connected to another TLP chassis and thecontroller modules are minimizing inter-TLP communications. Thisdecision making process is fully dynamic, but is assisted by tables inthe TLP 12 that pre-determine the preferred TLP chassis for everyprimary channel assignment. The tables are created by the operator ofthe Wireless Location System, and programmed using the NetworkOperations Console.

[0163] The networking described herein functions for both TLP chassisassociated with the same wireless carrier, as well as for chassis thatoverlap or border the coverage area between two wireless carriers. Thusit is possible for a TLP 12 belonging to a first wireless carrier to benetworked and therefore receive RF data from a TLP 12 (and the SCS's 10associated with that TLP 12) belonging to a second wireless carrier.This networking is particularly valuable in rural areas, wherein theperformance of the Wireless Location System can be enhanced by deployingSCS's 10 at cell sites of multiple wireless carriers. Since in manycases wireless carriers do not colocate cell sites, this feature enablesthe Wireless Location System to access more geographically diverseantennas than might be available if the Wireless Location System usedonly the cell sites from a single wireless carrier. As described below,the proper selection and use of antennas for location processing canenhance the performance of the Wireless Location System.

[0164] The controller module 12-3 passes many messages, includinglocation records, to the AP 14, many of which are described below.Usually, however, demodulated data is not passed from the TLP 12 to theAP 14. If, however, the TLP 12 receives demodulated data from aparticular wireless transmitter and the TLP 12 identifies the wirelesstransmitter as being a registered customer of a second wireless carrierin a different coverage area, the TLP 12 may pass the demodulated datato the first (serving) AP 14A. This will enable the first AP 14A tocommunicate with a second AP 14B associated with the second wirelesscarrier, and determine whether the particular wireless transmitter hasregistered for any type of location services. If so, the second AP 14Bmay instruct the first AP 14A to place the identity of the particularwireless transmitter into the Signal of Interest Table so that theparticular wireless transmitter will be located for as long as theparticular wireless transmitter is in the coverage area of the firstWireless Location System associated with the first AP 14A. When thefirst Wireless Location System has detected that the particular wirelesstransmitter has not registered in a time period exceeding apre-determined threshold, the first AP 14A may instruct the second AP14B that the identity of the particular wireless transmitter is beingremoved from the Signal of Interest Table for the reason of no longerbeing present in the coverage area associated with the first AP 14A.

[0165] Diagnostic Port

[0166] The TLP 12 supports a diagnostic port that is highly useful inthe operation and diagnosis of problems within the Wireless LocationSystem. This diagnostic port can be accessed either locally at a TLP 12or remotely over the Ethernet network connecting the TLP's 12 to theAP's. The diagnostic port enables an operator to write to a file all ofthe demodulation and RF data received from the SCS's 10, as well as theintermediate and final results of all location processing. This data iserased from the TLP 12 after processing a location estimate, andtherefore the diagnostic port provides the means to save the data forlater post-processing and analysis. The inventor's experience inoperating large scale wireless location systems is that a very smallnumber of location estimates can occasionally have very large errors,and these large errors can dominate the overall operating statistics ofthe Wireless Location System over any measurement period. Therefore, itis important to provide the operator with a set of tools that enable theWireless Location System to detect and trap the cause of the very largeerrors to diagnose and mitigate those errors. The diagnostic port can beset to save the above information for all location estimates, forlocation estimates from particular wireless transmitters or atparticular test points, or for location estimates that meet a certaincriteria. For example, for fixed phones or drive testing of surveyedpoints, the diagnostic port can determine the error in the locationestimate in real time and then write the above described informationonly for those location estimates whose error exceeds a predeterminedthreshold. The diagnostic port determines the error in real time bystoring the surveyed latitude, longitude coordinate of each fixed phoneand drive test point in a table, and then calculating a radial errorwhen a location estimate for the corresponding test point is made.

[0167] Redundancy

[0168] The TLP's 12 implement redundancy using several inventivetechniques, allowing the Wireless Location System to support an M plus Nredundancy method. M plus N redundancy means that N redundant (orstandby) TLP chassis are used to provide full redundant backup to Monline TLP chassis. For example, M may be ten and N may be two.

[0169] First, the controller modules in different TLP chassiscontinuously exchange status and “heartbeat” messages at pre-determinedtime intervals between themselves and with every AP 14 assigned tomonitor the TLP chassis. Thus, every controller module has continuousand full status of every other controller module in the WirelessLocation System. The controller modules in different TLP chassisperiodically select one controller module in one TLP 12 to be the mastercontroller for a group of TLP chassis. The master controller may decideto place a first TLP chassis into off-line status if the first TLP 12Areports a failed or degraded condition in its status message, or if thefirst TLP 12A fails to report any status or heartbeat messages withinits assigned and pre-determined time. If the master controller places afirst TLP 12A into off-line status, the master controller may assign asecond TLP 12B to perform a redundant switchover and assume the tasks ofthe off-line first TLP 12A. The second TLP 12B is automatically sent theconfiguration that had been loaded into the first TLP 12A; thisconfiguration may be downloaded from either the master controller orfrom an AP 14 connected to the TLP's 12. The master controller may be acontroller module on any one of the TLP's 12 that is not in off-linestatus, however there is a preference that the master controller be acontroller module in a stand-by TLP 12. When the master controller isthe controller module in a stand-by TLP 12, the time required to detecta failed first TLP 12A, place the first TLP 12A into off-line status,and then perform a redundant switchover can be accelerated.

[0170] Second, all of the T1 or E1 communications between the SCS's 10and each of the TLP T1/E1 communications modules 12-2 are preferablyrouted through a high-reliability DACS that is dedicated to redundancycontrol. The DACS 13B is connected to every groomed T1/E1 circuitcontaining DSO's from SCS's 10 and is also connected to every T1/E1communications module 12-2 of every TLP 12. Every controller module atevery TLP 12 contains a map of the DACS 13B that describes the DACS'connection list and port assignments. This DACS 13B is connected to theEthernet network described above and can be controlled by any of thecontroller modules 12-3 at any of the TLP's 12. When a second TLP 12 isplaced into off-line status by a master controller, the mastercontroller sends commands to the DACS 13B to switch the groomed T1/E1circuit communicating with the first TLP 12A to a second TLP 12B whichhad been in standby status. At the same time, the AP 14 downloads thecomplete configuration file that was being used by the second (and nowoff-line) TLP 12B to the third (and now online) TLP 12C. The time fromthe first detection of a failed first TLP chassis to the completeswitch-over and assumption of processing responsibilities by a third TLPchassis is typically less than few seconds. In many cases, no RF data islost by the SCS's 10 associated with the failed first TLP chassis, andlocation processing can continue without interruption. At the time of aTLP fail-over when a first TLP 12A is placed into off-line status, theNOC 16 creates an alert to notify the Wireless Location System operatorthat the event has occurred.

[0171] Third, each TLP chassis contains redundant power supplies, fans,and other components. A TLP chassis can also support multiple DSPmodules, so that the failure of a single DSP module or even a single DSPon a DSP module reduces the overall amount of processing resourcesavailable but does not cause the failure of the TLP chassis. In all ofthe cases described in this paragraph, the failed component of the TLP12 can be replaced without placing the entire TLP chassis into off-linestatus. For example, if a single power supply fails, the redundant powersupply has sufficient capacity to singly support the load of thechassis. The failed power supply contains the necessary circuitry toremove itself from the load of the chassis and not cause furtherdegradation in the chassis. Similarly, a failed DSP module can alsoremove itself from the active portions of the chassis, so as to notcause a failure of the backplane or other modules. This enables theremainder of the chassis, including the second DSP module, to continueto function normally. Of course, the total processing throughput of thechassis is reduced but a total failure is avoided.

[0172] Application Processor (AP) 14

[0173] The AP 14 is a centralized database system, comprising a numberof software processes that manage the entire Wireless Location System,provide interfaces to external users and applications, store locationrecords and configurations, and support various application-relatedfunctionality. The AP 14 uses a commercial hardware platform that issized to match the throughput of the Wireless Location System. The AP 14also uses a commercial relational database system (RDBMS), which hasbeen significantly customized to provide the functionality describedherein. While the SCS 10 and TLP 12 preferably operate together on apurely real time basis to determine location and create locationrecords, the AP 14 can operate on both a real time basis to store andforward location records and a non-real time basis to post-processlocation records and provide access and reporting over time. The abilityto store, retrieve, and post-process location records for various typesof system and application analysis has proven to be a powerful advantageof the present invention. The main collection of software processes isknown as the ApCore, which is shown in FIG. 4 and includes the followingfunctions:

[0174] The AP Performance Guardian (ApPerfGuard) is a dedicated softwareprocess that is responsible for starting, stopping, and monitoring mostother ApCore processes as well as ApCore communications with the NOC 16.Upon receiving a configuration update command from the NOC, ApPerfGuardupdates the database and notifies all other processes of the change.ApPerfGuard starts and stops appropriate processes when the NOC directsthe ApCore to enter specific run states, and constantly monitors othersoftware processes scheduled to be running to restart them if they haveexited or stopping and restarting any process that is no longer properlyresponding. ApPerfGuard is assigned to one of the highest processingpriorities so that this process cannot be blocked by another processthat has “run away”. ApPerfGuard is also assigned dedicated memory thatis not accessible by other software processes to prevent any possiblecorruption from other software processes.

[0175] The AP Dispatcher (ApMnDsptch) is a software process thatreceives location records from the TLP's 12 and forwards the locationrecords to other processes. This process contains a separate thread foreach physical TLP 12 configured in the system, and each thread receiveslocation records from that TLP 12. For system reliability, the ApCoremaintains a list containing the last location record sequence numberreceived from each TLP 12, and sends this sequence number to the TLP 12upon initial connection. Thereafter, the AP 14 and the TLP 12 maintain aprotocol whereby the TLP 12 sends each location record with a uniqueidentifier. ApMnDsptch forwards location records to multiple processes,including Ap911, ApDbSend, ApDbRecvLoc, and ApDbFileRecv.

[0176] The AP Tasking Process (ApDbSend) controls the Tasking Listwithin the Wireless Location System. The Tasking List is the master listof all of the trigger criteria that determines which wirelesstransmitters will be located, which applications created the criteria,and which applications can receive location record information. TheApDbSend process contains a separate thread for each TLP 12, over whichthe ApDbSend synchronizes the Tasking List with the Signal of InterestTable on each TLP 12. ApDbSend does not send application information tothe Signal of Interest Table, only the trigger criteria. Thus the TLP 12does not know why a wireless transmitter must be located. The TaskingList allows wireless transmitters to be located based upon MobileIdentity Number (MIN), Mobile Station Identifier (MSID), ElectronicSerial Number (ESN) and other identity numbers, dialed sequences ofcharacters and/or digits, home System ID (SID), originating cell siteand sector, originating RF channel, or message type. The Tasking Listallows multiple applications to receive location records from the samewireless transmitter. Thus, a single location record from a wirelesstransmitter that has dialed “911” can be sent, for example, to a 911PSAP, a fleet management application, a traffic management application,and to an RF optimization application.

[0177] The Tasking List also contains a variety of flags and field foreach trigger criteria, some of which are described elsewhere in thisspecification. One flag, for example, specifies the maximum time limitbefore which the Wireless Location System must provide a rough or finalestimate of the wireless transmitter. Another flag allows locationprocessing to be disabled for a particular trigger criteria such as theidentity of the wireless transmitter. Another field contains theauthentication required to make changes to the criteria for a particulartrigger; authentication enables the operator of the Wireless LocationSystem to specify which applications are authorized to add, delete, ormake changes to any trigger criteria and associated fields or flags.Another field contains the Location Grade of Service associated with thetrigger criteria; Grade of Service indicates to the Wireless LocationSystem the accuracy level and priority level desired for the locationprocessing associated with a particular trigger criteria. For example,some applications may be satisfied with a rough location estimate(perhaps for a reduced location processing fee), while otherapplications may be satisfied with low priority processing that is notguaranteed to complete for any given transmission (and which may bepreempted for high priority processing tasks). The Wireless LocationSystem also includes means to support the use of wildcards for triggercriteria in the Tasking List. For example, a trigger criteria can beentered as “MIN=215555****”. This will cause the Wireless LocationSystem to trigger location processing for any wireless transmitter whoseMIN begins with the six digits 215555 and ends with any following fourdigits. The wildcard characters can be placed into any position in atrigger criteria. This feature can save on the number of memorylocations required in the Tasking List and Signal of Interest Table bygrouping blocks of related wireless transmitters together.

[0178] ApDbSend also supports dynamic tasking. For example, the MIN,ESN, MSID, or other identity of any wireless transmitter that has dialed“911” will automatically be placed onto the Tasking List by ApDbSend forone hour. Thus, any further transmissions by the wireless transmitterthat dialed “911” will also be located in case of further emergency. Forexample, if a PSAP calls back a wireless transmitter that had dialed“911” within the last hour, the Wireless Location System will trigger onthe page response message from the wireless transmitter, and can makethis new location record available to the PSAP. This dynamic tasking canbe set for any interval of time after an initiation event, and for anytype of trigger criteria. The ApDbSend process is also a server forreceiving tasking requests from other applications. These applications,such as fleet management, can send tasking requests via a socketconnection, for example. These applications can either place or removetrigger criteria. ApDbSend conducts an authentication process with eachapplication to verify that that the application has been authorized toplace or remove trigger criteria, and each application can only changetrigger criteria related to that application.

[0179] The AP 911 Process (Ap911) manages each interface between theWireless Location System and E9-1-1 network elements, such as tandemswitches, selective routers, ALI databases and/or PSAPs. The Ap911process contains a separate thread for each connection to a E9-1-1network element, and can support more than one thread to each networkelement. The Ap911 process can simultaneously operate in many modesbased upon user configuration, and as described herein. The timelyprocessing of E9-1-1 location records is one of the highest processingpriorities in the AP 14, and therefore the Ap911 executes entirely outof random access memory (RAM) to avoid the delay associated with firststoring and then retrieving a location record from any type of disk.When ApMnDsptch forwards a location record to Ap911, Ap911 immediatelymakes a routing determination and forwards the location record over theappropriate interface to a E9-1-1 network element. A separate process,operating in parallel, records the location record into the AP 14database.

[0180] The AP 14, through the Ap911 process and other processes,supports two modes of providing location records to applications,including E9-1-1: “push” and “pull” modes. Applications requesting pushmode receive a location record as soon as it is available from the AP14. This mode is especially effective for E9-1-1 which has a very timecritical need for location records, since E9-1-1 networks must routewireless 9-1-1 calls to the correct PSAP within a few seconds after awireless caller has dialed “911”. Applications requesting pull mode donot automatically receive location records, but rather must send a queryto the AP 14 regarding a particular wireless transmitter in order toreceive the last, or any other location record, about the wirelesstransmitter. The query from the application can specify the lastlocation record, a series of location records, or all location recordsmeeting a specific time or other criteria, such as type of transmission.An example of the use of pull mode in the case of a “911” call is theE9-1-1 network first receiving the voice portion of the “911” call andthen querying the AP 14 to receive the location record associated withthat call.

[0181] When the Ap911 process is connected to many E9-1-1 networkselements, Ap911 must determine to which E9-1-1 network element to pushthe location record (assuming that “push” mode has been selected). TheAP 14 makes this determination using a dynamic routing table. Thedynamic routing table is used to divide a geographic region into cells.Each cell, or entry, in the dynamic routing table contains the routinginstructions for that cell. It is well known that one minute of latitudeis 6083 feet, which is about 365 feet per millidegree. Additionally, oneminute of longitude is cosine(latitude) times 6083 feet, which for thePhiladelphia area is about 4659 feet, or about 280 feet per millidegree.A table of size one thousand by one thousand, or one million cells, cancontain the routing instructions for an area that is about 69 miles by53 miles, which is larger than the area of Philadelphia in this example,and each cell could contain a geographic area of 365 feet by 280 feet.The number of bits allocated to each entry in the table must only beenough to support the maximum number of routing possibilities. Forexample, if the total number of routing possibilities is sixteen orless, then the memory for the dynamic routing table is one million timesfour bits, or one-half megabyte. Using this scheme, an area the size ofPennsylvania could be contained in a table of approximately twentymegabytes or less, with ample routing possibilities available. Given therelatively inexpensive cost of memory, this inventive dynamic routingtable provides the AP 14 with a means to quickly push the locationrecords for “911” calls only to the appropriate E9-1-1 network element.

[0182] The AP 14 allows each entry in dynamic routing to be populatedusing manual or automated means. Using the automated means, for example,an electronic map application can create a polygon definition of thecoverage area of a specific E9-1-1 network element, such as a PSAP. Thepolygon definition is then translated into a list of latitude, longitudepoints contained within the polygon. The dynamic routing table cellcorresponding to each latitude, longitude point is then given therouting instruction for that E9-1-1 network element that is responsiblefor that geographic polygon.

[0183] When the Ap911 process receives a “911” location record for aspecific wireless transmitter, Ap911 converts the latitude, longitudeinto the address of a specific cell in the dynamic routing table. Ap911then queries the cell to determine the routing instructions, which maybe push or pull mode and the identity of the E9-1-1 network elementresponsible for serving the geographic area in which the “911” calloccurred. If push mode has been selected, then Ap911 automaticallypushes the location record to that E9-1-1 network element. If pull modehas been selected, then Ap911 places the location record into a circulartable of “911” location records and awaits a query.

[0184] The dynamic routing means described above entails the use of ageographically defined database that may be applied to otherapplications in addition to 911, and is therefore supported by otherprocesses in addition to Ap911. For example, the AP 14 can automaticallydetermine the billing zone from which a wireless call was placed for aLocation Sensitive Billing application. In addition, the AP 14 mayautomatically send an alert when a particular wireless transmitter hasentered or exited a prescribed geographic area defined by anapplication. The use of particular geographic databases, dynamic routingactions, any other location triggered actions are defined in the fieldsand flags associated with each trigger criteria. The Wireless LocationSystem includes means to easily manage these geographically defineddatabases using an electronic map that can create polygons encompassinga prescribed geographic area. The Wireless Location System extracts fromthe electronic map a table of latitude, longitude points contained withthe polygon. Each application can use its own set of polygons, and candefine a set of actions to be taken when a location record for atriggered wireless transmission is contained within each polygon in theset.

[0185] The AP Database Receive Process (ApDbRecvLoc) receives alllocation records from ApMnDsptch via shared memory, and places thelocation records into the AP location database. ApDbRecvLoc starts tenthreads that each retrieve location records from shared memory, validateeach record before inserting the records into the database, and theninserts the records into the correct location record partition in thedatabase. To preserve integrity, location records with any type of errorare not written into the location record database but are instead placedinto an error file that can be reviewed by the Wireless Location Systemoperator and then manually entered into the database after errorresolution. If the location database has failed or has been placed intooff-line status, location records are written to a flat file where theycan be later processed by ApDbFileRecv.

[0186] The AP File Receive Process (ApDbFileRecv) reads flat filescontaining location records and inserts the records into the locationdatabase. Flat files are a safe mechanism used by the AP 14 tocompletely preserve the integrity of the AP 14 in all cases except acomplete failure of the hard disk drives. There are several differenttypes of flat files read by ApDbFileRecv, including Database Down,Synchronization, Overflow, and Fixed Error. Database Down flat files arewritten by the ApDbRecvLoc process if the location database istemporarily inaccessible; this file allows the AP 14 to ensure thatlocation records are preserved during the occurrence of this type ofproblem. Synchronization flat files are written by the ApLocSync process(described below) when transferring location records between pairs ofredundant AP systems. Overflow flat files are written by ApMnDsptch whenlocation records are arriving into the AP 14 at a rate faster thanApDbRecvLoc can process and insert the records into the locationdatabase. This may occur during very high peak rate periods. Theoverflow files prevent any records from being lost during peak periods.The Fixed Error flat files contain location records that had errors buthave now been fixed, and can now be inserted into the location database.

[0187] Because the AP 14 has a critical centralized role in the WirelessLocation System, the AP 14 architecture has been designed to be fullyredundant. A redundant AP 14 system includes fully redundant hardwareplatforms, fully redundant RDBMS, redundant disk drives, and redundantnetworks to each other, the TLP's 12, the NOC's 16, and externalapplications. The software architecture of the AP 14 has also beendesigned to support fault tolerant redundancy. The following examplesillustrate functionality supported by the redundant AP's. Each TLP 12sends location records to both the primary and the redundant AP 14 whenboth AP's are in an online state. Only the primary AP 14 will processincoming tasking requests, and only the primary AP 14 will acceptconfiguration change requests from the NOC 16. The primary AP 14 thensynchronizes the redundant AP 14 under careful control. Both the primaryand redundant AP's will accept basic startup and shutdown commands fromthe NOC. Both AP's constantly monitor their own system parameters andapplication health and monitor the corresponding parameters for theother AP 14, and then decide which AP 14 will be primary and which willbe redundant based upon a composite score. This composite score isdetermined by compiling errors reported by various processes to a sharedmemory area, and monitoring swap space and disk space. There are severalprocesses dedicated to supporting redundancy.

[0188] The AP Location Synchronization Process (ApLocSync) runs on eachAP 14 and detects the need to synchronize location records between AP's,and then creates “sync records” that list the location records that needto be transferred from one AP 14 to another AP 14. The location recordsare then transferred between AP's using a socket connection. ApLocSynccompares the location record partitions and the location record sequencenumbers stored in each location database. Normally, if both the primaryand redundant AP 14 are operating properly, synchronization is notneeded because both AP's are receiving location records simultaneouslyfrom the TLP's 12. However, if one AP 14 fails or is placed in anoff-line mode, then synchronization will later be required. ApLocSync isnotified whenever ApMnDsptch connects to a TLP 12 so it can determinewhether synchronization is required.

[0189] The AP Tasking Synchronization Process (ApTaskSync) runs on eachAP 14 and synchronizes the tasking information between the primary AP 14and the redundant AP 14. ApTaskSync on the primary AP 14 receivestasking information from ApDbSend, and then sends the taskinginformation to the ApTaskSync process on the redundant AP 14. If theprimary AP 14 were to fail before ApTaskSync had completed replicatingtasks, then ApTaskSync will perform a complete tasking databasesynchronization when the failed AP 14 is placed back into an onlinestate.

[0190] The AP Configuration Synchronization Process (ApConfigSync) runson each AP 14 and synchronizes the configuration information between theprimary AP 14 and the redundant AP 14. ApConfigSync uses a RDBMSreplication facility. The configuration information includes allinformation needed by the SCS's 10, TLP's 12, and AP's 14 for properoperation of the Wireless Location System in a wireless carrier'snetwork.

[0191] In addition to the core functions described above, the AP 14 alsosupports a large number of processes, functions, and interfaces usefulin the operation of the Wireless Location System, as well as useful forvarious applications that desire location information. While theprocesses, functions, and interfaces described herein are in thissection pertaining to the AP 14, the implementation of many of theseprocesses, functions, and interfaces permeates the entire WirelessLocation System and therefore their inventive value should be not readas being limited only to the AP 14.

[0192] Roaming

[0193] The AP 14 supports “roaming” between wireless location systemslocated in different cities or operated by different wireless carriers.If a first wireless transmitter has subscribed to an application on afirst Wireless Location System, and therefore has an entry in theTasking List in the first AP 14 in the first Wireless Location System,then the first wireless transmitter may also subscribe to roaming. EachAP 14 and TLP 12 in each Wireless Location System contains a table inwhich a list of valid “home” subscriber identities is maintained. Thelist is typically a range, and for example, for current cellulartelephones, the range can be determined by the NPA/NXX codes (or areacode and exchange) associated with the MIN or MSID of cellulartelephones. When a wireless transmitter meeting the “home” criteriamakes a transmission, a TLP 12 receives demodulated data from one ormore SCS's 10 and checks the trigger information in the Signal ofInterest Table. If any trigger criterion is met, the location processingbegins on that transmission; otherwise, the transmission is notprocessed by the Wireless Location System.

[0194] When a first wireless transmitter not meeting the “home”criterion makes a transmission in a second Wireless Location System, thesecond TLP 12 in the second Wireless Location System checks the Signalof Interest Table for a trigger. One of three actions then occurs: (i)if the transmission meets an already existing criteria in the Signal ofInterest Table, the transmitter is located and the location record isforwarded from the second AP 14 in the second Wireless Location Systemto the first AP 14 in the first Wireless Location System; (ii) if thefirst wireless transmitter has a “roamer” entry in the Signal ofInterest Table indicating that the first wireless transmitter has“registered” in the second Wireless Location System but has no triggercriteria, then the transmission is not processed by the second WirelessLocation System and the expiration timestamp is adjusted as describedbelow; (iii) if the first wireless transmitter has no “roamer” entry andtherefore has not “registered”, then the demodulated data is passed fromthe TLP 12 to the second AP 14.

[0195] In the third case above, the second AP 14 uses the identity ofthe first wireless transmitter to identify the first AP 14 in the firstWireless Location System as the “home” Wireless Location System of thefirst wireless transmitter. The second AP 14 in the second WirelessLocation System sends a query to the first AP 14 in the first WirelessLocation System to determine whether the first wireless transmitter hassubscribed to any location application and therefore has any triggercriteria in the Tasking List of the first AP 14. If a trigger is presentin the first AP 14, the trigger criteria, along with any associatedfields and flags, is sent from the first AP 14 to the second AP 14 andentered in the Tasking List and the Signal of Interest Table as a“roamer” entry with trigger criteria. If the first AP 14 responds to thesecond AP 14 indicating that the first wireless transmitter has notrigger criteria, then the second AP 14 “registers” the first wirelesstransmitter in the Tasking List and the Signal of Interest Table as a“roamer” with no trigger criteria. Thus both current and futuretransmissions from the first wireless transmitter can be positivelyidentified by the TLP 12 in the second Wireless Location System as beingregistered without trigger criteria, and the second AP 14 is notrequired to make additional queries to the first AP 14.

[0196] When the second AP 14 registers the first wireless transmitterwith a roamer entry in the Tasking List and the Signal of Interest Tablewith or without trigger criteria, the roamer entry is assigned anexpiration timestamp. The expiration timestamp is set to the currenttime plus a predetermined first interval. Every time the first wirelesstransmitter makes a transmission, the expiration timestamp of the roamerentry in the Tasking List and the Signal of Interest Table is adjustedto the current time of the most recent transmission plus thepredetermined first interval. If the first wireless transmitter makes nofurther transmissions prior to the expiration timestamp of its roamerentry, then the roamer entry is automatically deleted. If, subsequent tothe deletion, the first wireless transmitter makes another transmission,then the process of registering occurs again.

[0197] The first AP 14 and second AP 14 maintain communications over awide area network. The network may be based upon TCP/IP or upon aprotocol similar to the most recent version of IS-41. Each AP 14 incommunications with other AP's in other wireless location systemsmaintains a table that provides the identity of each AP 14 and WirelessLocation System corresponding to each valid range of identities ofwireless transmitters.

[0198] Multiple Pass Location Records

[0199] Certain applications may require a very fast estimate of thegeneral location of a wireless transmitter, followed by a more accurateestimate of the location that can be sent subsequently. This can bevaluable, for example, for E9-1-1 systems that handle wireless calls andmust make a call routing decision very quickly, but can wait a littlelonger for a more exact location to be displayed upon the E9-1-1call-taker's electronic map terminal. The Wireless Location Systemsupports these applications with an inventive multiple pass locationprocessing mode, described later. The AP 14 supports this mode withmultiple pass location records. For certain entries, the Tasking List inthe AP 14 contains a flag indicating the maximum time limit before whicha particular application must receive a rough estimate of location, anda second maximum time limit in which a particular application mustreceive a final location estimate. For these certain applications, theAP 14 includes a flag in the location record indicating the status ofthe location estimate contained in the record, which may, for example,be set to first pass estimate (i.e. rough) or final pass estimate. TheWireless Location System will generally determine the best locationestimate within the time limit set by the application, that is theWireless Location System will process the most amount of RF data thatcan be supported in the time limit. Given that any particular wirelesstransmission can trigger a location record for one or more applications,the Wireless Location System supports multiple modes simultaneously. Forexample, a wireless transmitter with a particular MIN can dial “911”.This may trigger a two-pass location record for the E9-1-1 application,but a single pass location record for a fleet management applicationthat is monitoring that particular MIN. This can be extended to anynumber of applications.

[0200] Multiple Demodulation and Triggers

[0201] In wireless communications systems in urban or dense suburbanareas, frequencies or channels can be re-used several times withinrelatively close distances. Since the Wireless Location System iscapable of independently detecting and demodulating wirelesstransmissions without the aid of the wireless communications system, asingle wireless transmission can frequently be detected and successfullydemodulated at multiple SCS's 10 within the Wireless Location System.This can happen both intentionally and unintentionally. An unintentionaloccurrence is caused by a close frequency re-use, such that a particularwireless transmission can be received above a predetermined threshold atmore than one SCS 10, when each SCS 10 believes it is monitoring onlytransmissions that occur only within the cell site collocated with theSCS 10. An intentional occurrence is caused by programming more than oneSCS 10 to detect and demodulate transmissions that occur at a particularcell site and on a particular frequency. As described earlier, this isgenerally used with adjacent or nearby SCS's 10 to provide systemdemodulation redundancy to further increase the probability that anyparticular wireless transmission is successful detected and demodulated.

[0202] Either type of event could potentially lead to multiple triggerswithin the Wireless Location System, causing location processing to beinitiated several times for the same transmission. This causes an excessand inefficient use of processing and communications resources.Therefore, the Wireless Location System includes means to detect whenthe same transmission has been detected and demodulated more than once,and to select the best demodulating SCS 10 as the starting point forlocation processing. When the Wireless Location System detects andsuccessfully demodulates the same transmission multiple times atmultiple SCS/antennas, the Wireless Location System uses the followingcriteria to select the one demodulating SCS/antenna to use to continuethe process of determining whether to trigger and possibly initiatelocation processing (again, these criteria may be weighted indetermining the final decision): (i) an SCS/antenna collocated at thecell site to which a particular frequency has been assigned is preferredover another SCS/antenna, but this preference may be adjusted if thereis no operating and on-line SCS/antenna collocated at the cell site towhich the particular frequency has been assigned, (ii) SCS/antennas withhigher average SNR are preferred over those with lower average SNR, and(iii) SCS/antennas with fewer bit errors in demodulating thetransmission are preferred over those with higher bit errors. Theweighting applied to each of these preferences may be adjusted by theoperator of the Wireless Location System to suit the particular designof each system.

[0203] Interface to Wireless Communications System

[0204] The Wireless Location System contains means to communicate overan interface to a wireless communications system, such as a mobileswitching center (MSC) or mobile positioning controller (MPC). Thisinterface may be based, for example, on a standard secure protocol suchas the most recent version of the IS-41 or TCP/IP protocols. Theformats, fields, and authentication aspects of these protocols are wellknown. The Wireless Location System supports a variety ofcommand/response and informational messages over this interface that aredesigned to aid in the successful detection, demodulation, andtriggering of wireless transmissions, as well as providing means to passlocation records to the wireless communications system. In particular,this interface provides means for the Wireless Location System to obtaininformation about which wireless transmitters have been assigned toparticular voice channel parameters at particular cell sites. Examplemessages supported by the Wireless Location System over this interfaceto the wireless communications system include the following:

[0205] Query on MIN/MDN/MSID/IMSI/TMSI Mapping—Certain types of wirelesstransmitters will transmit their identity in a familiar form that can bedialed over the telephone network. Other types of wireless transmitterstransmit an identity that cannot be dialed, but which is translated intoa number that can be dialed using a table inside of the wirelesscommunications system. The transmitted identity is permanent in mostcases, but can also be temporary. Users of location applicationsconnected to the AP 14 typically prefer to place triggers onto theTasking List using identities that can be dialed. Identities that can bedialed are typically known as Mobile Directory Numbers (MDN). The othertypes of identities for which translation may be required includesMobile Identity Number (MIN), Mobile Subscriber Identity (MSID),International Mobile Subscriber Identity (IMSI), and Temporary MobileSubscriber Identity (TMSI). If the wireless communications system hasenabled the use of encryption for any of the data fields in the messagestransmitted by wireless transmitters, the Wireless Location System mayalso query for encryption information along with the identityinformation. The Wireless Location System includes means to query thewireless communications system for the alternate identities for atrigger identity that has been placed onto the Tasking List by alocation application, or to query the wireless communications system foralternate identities for an identity that has been demodulated by an SCS10. Other events can also trigger this type of query. For this type ofquery, typically the Wireless Location System initiates the command, andthe wireless communications system responds.

[0206] Query/Command Change on Voice RF Channel Assignment—Many wirelesstransmissions on voice channels do not contain identity information.Therefore, when the Wireless Location System is triggered to performlocation processing on a voice channel transmission, the WirelessLocation System queries the wireless communication system to obtain thecurrent voice channel assignment information for the particulartransmitter for which the Wireless Location System has been triggered.For an AMPS transmission, for example, the Wireless Location Systempreferably requires the cell site, sector, and RF channel numbercurrently in use by the wireless transmitter. For a TDMA transmission,for example, the Wireless Location System preferably requires the cellsite, sector, RF channel number, and timeslot currently in use by thewireless transmitter. Other information elements that may be neededincludes long code mask and encryption keys. In general, the WirelessLocation System will initiate the command, and the wirelesscommunications system will respond. However, the Wireless LocationSystem will also accept a trigger command from the wirelesscommunications system that contains the information detailed herein.

[0207] The timing on this command/response message set is very criticalsince voice channel handoffs can occur quite frequently in wirelesscommunications systems. That is, the Wireless Location System willlocate any wireless transmitter that is transmitting on a particularchannel—therefore the Wireless Location System and the wirelesscommunications system must jointly be certain that the identity of thewireless transmitter and the voice channel assignment information are inperfect synchronization. The Wireless Location System uses several meansto achieve this objective. The Wireless Location System may, forexample, query the voice channel assignment information for a particularwireless transmitter, receive the necessary RF data, then again querythe voice channel assignment information for that same wirelesstransmitter, and then verify that the status of the wireless transmitterdid not change during the time in which the RF data was being collectedby the Wireless Location System. Location processing is not required tocomplete before the second query, since it is only important to verifythat the correct RF data was received. The Wireless Location System mayalso, for example, as part of the first query command the wirelesscommunications system to prevent a handoff from occurring for theparticular wireless transmitter during the time period in which theWireless Location System is receiving the RF data. Then, subsequent tocollecting the RF data, the Wireless Location System will again querythe voice channel assignment information for that same wirelesstransmitter, command the wireless communications system to again permithandoffs for said wireless transmitter and then verify that the statusof the wireless transmitter did not change during the time in which theRF data was being collected by the Wireless Location System.

[0208] For various reasons, either the Wireless Location System or thewireless communications system may prefer that the wireless transmitterbe assigned to another voice RF channel prior to performing locationprocessing. Therefore, as part of the command/response sequence, thewireless communications system may instruct the Wireless Location Systemto temporarily suspend location processing until the wirelesscommunications system has completed a handoff sequence with the wirelesstransmitter, and the wireless communications system has notified theWireless Location System that RF data can be received, and the voice RFchannel upon which the data can be received. Alternately, the WirelessLocation System may determine that the particular voice RF channel whicha particular wireless transmitter is currently using is unsuitable forobtaining an acceptable location estimate, and request that the wirelesscommunications system command the wireless transmitter to handoff.Alternately, the Wireless Location System may request that the wirelesscommunications system command the wireless transmitter to handoff to aseries of voice RF channels in sequence in order to perform a series oflocation estimates, whereby the Wireless Location System can improveupon the accuracy of the location estimate through the series ofhandoffs; this method is further described later. The Wireless LocationSystem can also use this command/response message set to query thewireless communications system about the identity of a wirelesstransmitter that had been using a particular voice channel (andtimeslot, etc.) at a particular cell site at a particular time. Thisenables the Wireless Location System to first perform locationprocessing on transmissions without knowing the identities, and then tolater determine the identity of the wireless transmitters making thetransmissions and append this information to the location record. Thisparticular inventive feature enables the use of automatic sequentiallocation of voice channel transmissions.

[0209] Receive Triggers—The Wireless Location System can receivetriggers from the wireless communications system to perform locationprocessing on a voice channel transmission without knowing the identityof the wireless transmitter. This message set bypasses the Tasking List,and does not use the triggering mechanisms within the Wireless LocationSystem. Rather, the wireless communications system alone determineswhich wireless transmissions to locate, and then send a command to theWireless Location System to collect RF data from a particular voicechannel at a particular cell site and to perform location processing.The Wireless Location System responds with a confirmation containing atimestamp when the RF data was collected. The Wireless Location Systemalso responds with an appropriate format location record when locationprocessing has completed. Based upon the time of the command to WirelessLocation System and the response with the RF data collection timestamp,the wireless communications system determines whether the wirelesstransmitter status changed subsequent to the command and whether thereis a good probability of successful RF data collection.

[0210] Make Transmit—The Wireless Location System can command thewireless communications system to force a particular wirelesstransmitter to make a transmission at a particular time, or within aprescribed range of times. The wireless communications system respondswith a confirmation and a time or time range in which to expect thetransmission. The types of transmissions that the Wireless LocationSystem can force include, for example, audit responses and pageresponses. Using this message set, the Wireless Location System can alsocommand the wireless communications system to force the wirelesstransmitter to transmit using a higher power level setting. In manycases, wireless transmitters will attempt to use the lowest power levelsettings when transmitting in order to conserve battery life. In orderimprove the accuracy of the location estimate, the Wireless LocationSystem may prefer that the wireless transmitter use a higher power levelsetting. The wireless communications system will respond to the WirelessLocation System with a confirmation that the higher power level settingwill be used and a time or time range in which to expect thetransmission.

[0211] Delay Wireless Communications System Response to MobileAccess—Some air interface protocols, such as CDMA, use a mechanism inwhich the wireless transmitter initiates transmissions on a channel,such as an Access Channel, for example, at the lowest or a very lowpower level setting, and then enters a sequence of steps in which (i)the wireless transmitter makes an access transmission; (ii) the wirelesstransmitter waits for a response from the wireless communicationssystem; (iii) if no response is received by the wireless transmitterfrom the wireless communications system within a predetermined time, thewireless transmitter increases its power level setting by apredetermined amount, and then returns to step (i); (iv) if a responseis received by the wireless transmitter from the wireless communicationssystem within a predetermined time, the wireless transmitter then entersa normal message exchange. This mechanism is useful to ensure that thewireless transmitter uses only the lowest useful power level setting fortransmitting and does not further waste energy or battery life. It ispossible, however, that the lowest power level setting at which thewireless transmitter can successfully communicate with the wirelesscommunications system is not sufficient to obtain an acceptable locationestimate. Therefore, the Wireless Location System can command thewireless communications system to delay its response to thesetransmissions by a predetermined time or amount. This delaying actionwill cause the wireless transmitter to repeat the sequence of steps (i)through (iii) one or more times than normal with the result that one ormore of the access transmissions will be at a higher power level thannormal. The higher power level may preferably enable the WirelessLocation System to determine a more accurate location estimate. TheWireless Location System may command this type of delaying action foreither a particular wireless transmitter, for a particular type ofwireless transmission (for example, for all ‘911’ calls), for wirelesstransmitters that are at a specified range from the base station towhich the transmitter is attempting to communicate, or for all wirelesstransmitters in a particular area.

[0212] Send Confirmation to Wireless Transmitter—The Wireless LocationSystem does not include means within to notify the wireless transmitterof an action because the Wireless Location System cannot transmit; asdescribed earlier the Wireless Location System can only receivetransmissions. Therefore, if the Wireless Location System desires tosend, for example, a confirmation tone upon the completion of a certainaction, the Wireless Location System commands the wirelesscommunications system to transmit a particular message. The message mayinclude, for example, an audible confirmation tone, spoken message, orsynthesized message to the wireless transmitter, or a text message sentvia a short messaging service or a page. The Wireless Location Systemreceives confirmation from the wireless communications system that themessage has been accepted and sent to the wireless transmitter. Thiscommand/response message set is important in enabling the WirelessLocation System to support certain end-user application functions suchas Prohibit Location Processing.

[0213] Report Location Records—The Wireless Location Systemautomatically reports location records to the wireless communicationssystem for those wireless transmitters tasked to report to the wirelesscommunications system, as well as for those transmissions that thewireless communications system initiated triggers. The Wireless LocationSystem also reports on any historical location record queried by thewireless communications system and which the wireless communicationssystem is authorized to receive.

[0214] Monitor Internal Wireless Communications System Interfaces, StateTable

[0215] In addition to this above interface between the Wireless LocationSystem and the wireless communications system, the Wireless LocationSystem also includes means to monitor existing interfaces within thewireless communications system for the purpose of intercepting messagesimportant to the Wireless Location System for identifying wirelesstransmitters and the RF channels in use by these transmitters. Theseinterfaces may include, for example, the “a-interface” and “a-bisinterface” used in wireless communications systems employing the GSM airinterface protocol. These interfaces are well-known and published invarious standards. By monitoring the bi-directional messages on theseinterfaces between base stations (BTS), base station controllers (BSC),and mobile switching centers (MSC), and other points, the WirelessLocation System can obtain the same information about the assignment ofwireless transmitters to specific channels as the wirelesscommunications system itself knows. The Wireless Location Systemincludes means to monitor these interfaces at various points. Forexample, the SCS 10 may monitor a BTS to BSC interface. Alternately, aTLP 12 or AP 14 may also monitor a BSC where a number of BTS to BSCinterfaces have been concentrated. The interfaces internal to thewireless communications system are not encrypted and the layeredprotocols are known to those familiar with the art. The advantage to theWireless Location System to monitoring these interfaces is that theWireless Location System may not be required to independently detect anddemodulate control channel messages from wireless transmitters. Inaddition, the Wireless Location System may obtain all necessary voicechannel assignment information from these interfaces.

[0216] Using these means for a control channel transmission, the SCS 10receives the transmissions as described earlier and records the controlchannel RF data into memory without performing detection anddemodulation. Separately, the Wireless Location System monitors themessages occurring over prescribed interfaces within the wirelesscommunications system, and causes a trigger in the Wireless LocationSystem when the Wireless Location System discovers a message containinga trigger event. Initiated by the trigger event, the Wireless LocationSystem determines the approximately time at which the wirelesstransmission occurred, and commands a first SCS 10 and a second SCS 10Bto each search its memory for the start of transmission. This first SCS10A chosen is an SCS that is either collocated with the base station towhich the wireless transmitter had communicated, or an SCS which isadjacent to the base station to which the wireless transmitter hadcommunicated. That is, the first SCS 10A is an SCS which would have beenassigned the control channel as a primary channel. If the first SCS 10Asuccessfully determines and reports the start of the transmission, thenlocation processing proceeds normally, using the means described below.If the first SCS 10A cannot successfully determine the start oftransmission, then the second SCS 10B reports the start of transmission,and then location processing proceeds normally.

[0217] The Wireless Location System also uses these means for voicechannel transmissions. For all triggers contained in the Tasking List,the Wireless Location System monitors the prescribed interfaces formessages pertaining to those triggers. The messages of interest include,for example, voice channel assignment messages, handoff messages,frequency hopping messages, power up/power down messages, directedre-try messages, termination messages, and other similar action andstatus messages. The Wireless Location System continuously maintains acopy of the state and status of these wireless transmitters in a StateTable in the AP 14. Each time that the Wireless Location System detectsa message pertaining to one of the entries in the Tasking List, theWireless Location System updates its own State Table. Thereafter, theWireless Location System may trigger to perform location processing,such as on a regular time interval, and access the State Table todetermine precisely which cell site, sector, RF channel, and timeslot ispresently being used by the wireless transmitter. The example containedherein described the means by which the Wireless Location Systeminterfaces to a GSM based wireless communications system. The WirelessLocation System also supports similar functions with systems based uponother air interfaces.

[0218] For certain air interfaces, such as CDMA, the Wireless LocationSystem also keeps certain identity information obtained from Accessbursts in the control channel in the State Table; this information islater used for decoding the masks used for voice channels. For example,the CDMA air interface protocol uses the Electronic Serial Number (ESN)of a wireless transmitter to, in part, determine the long code mask usedin the coding of voice channel transmissions. The Wireless LocationSystem maintains this information in the State Table for entries in theTasking List because many wireless transmitters may transmit theinformation only once; for example, many CDMA mobiles will only transmittheir ESN during the first Access burst after the wireless transmitterbecome active in a geographic area. This ability to independentlydetermine the long code mask is very useful in cases where an interfacebetween the Wireless Location System and the wireless communicationssystem is not operative and/or the Wireless Location System is not ableto monitor one of the interfaces internal to the wireless communicationssystem. The operator of the Wireless Location System may optionally setthe Wireless Location System to maintain the identity information forall wireless transmitters. In addition to the above reasons, theWireless Location System can provide the voice channel tracking for allwireless transmitters that trigger location processing by calling “911”.As described earlier, the Wireless Location System uses dynamic taskingto provide location to a wireless transmitter for a prescribed timeafter dialing “911”, for example. By maintaining the identityinformation for all wireless transmitters in the State Table, theWireless Location System is able to provide voice channel tracking forall transmitters in the event of a prescribed trigger event, and notjust those with prior entries in the Tasking List.

[0219] Applications Interface

[0220] Using the AP 14, the Wireless Location System supports a varietyof standards based interfaces to end-user and carrier locationapplications using secure protocols such as TCP/IP, X.25, SS-7, andIS-41. Each interface between the AP 14 and an external application is asecure and authenticated connection that permits the AP 14 to positivelyverify the identity of the application that is connected to the AP 14.This is necessary because each connected application is granted onlylimited access to location records on a real-time and/or historicalbasis. In addition, the AP 14 supports additional command/response,real-time, and post-processing functions that are further detailedbelow. Access to these additional functions also requiresauthentication. The AP 14 maintains a user list and the authenticationmeans associated with each user. No application can gain access tolocation records or functions for which the application does not haveproper authentication or access rights. In addition, the AP 14 supportsfull logging of all actions taken by each application in the event thatproblems arise or a later investigation into actions is required. Foreach command or function in the list below, the AP 14 preferablysupports a protocol in which each action or the result of each isconfirmed, as appropriate.

[0221] Edit Tasking List—This command permits external applications toadd, remove, or edit entries in the Tasking List, including any fieldsand flags associated with each entry. This command can be supported on asingle entry basis, or a batch entry basis where a list of entries isincluded in a single command. The latter is useful, for example, in abulk application such as location sensitive billing whereby largervolumes of wireless transmitters are being supported by the externalapplication, and it is desired to minimize protocol overhead. Thiscommand can add or delete applications for a particular entry in theTasking List, however, this command cannot delete an entry entirely ifthe entry also contains other applications not associated with orauthorized by the application issuing the command.

[0222] Set Location Interval—The Wireless Location System can be set toperform location processing at any interval for a particular wirelesstransmitter, on either control or voice channels. For example, certainapplications may require the location of a wireless transmitter everyfew seconds when the transmitter is engaged on a voice channel. When thewireless transmitter make an initial transmission, the Wireless LocationSystem initially triggers using a standard entry in the Tasking List. Ifone of the fields or flags in this entry specifies updated location on aset interval, then the Wireless Location System creates a dynamic taskin the Tasking List that is triggered by a timer instead of an identityor other transmitted criteria. Each time the timer expires, which canrange from 1 second to several hours, the Wireless Location System willautomatically trigger to locate the wireless transmitter. The WirelessLocation System uses its interface to the wireless communications systemto query status of the wireless transmitter, including voice callparameters as described earlier. If the wireless transmitter is engagedon a voice channel, then the Wireless Location System performs locationprocessing. If the wireless transmitter is not engaged in any existingtransmissions, the Wireless Location System will command the wirelesscommunications system to make the wireless transmitter immediatelytransmit. When the dynamic task is set, the Wireless Location Systemalso sets an expiration time at which the dynamic task ceases.

[0223] End-User Addition/Deletion—This command can be executed by anend-user of a wireless transmitter to place the identity of the wirelesstransmitter onto the Tasking List with location processing enabled, toremove the identity of the wireless transmitter from the Tasking Listand therefore eliminate identity as a trigger, or to place the identityof the wireless transmitter onto the Tasking List with locationprocessing disabled. When location processing has been disabled by theend-user, known as Prohibit Location Processing then no locationprocessing will be performed for the wireless transmitter. The operatorof the Wireless Location System can optionally select one of severalactions by the Wireless Location System in response to a ProhibitLocation Processing command by the end user: (i) the disabling actioncan override all other triggers in the Tasking List, including a triggerdue to an emergency call such as “911”, (ii) the disabling action canoverride any other trigger in the Tasking List, except a trigger due toan emergency call such as “911”, (iii) the disabling action can beoverridden by other select triggers in the Tasking List. In the firstcase, the end-user is granted complete control over the privacy of thetransmissions by the wireless transmitter, as no location processingwill be performed on that transmitter for any reason. In the secondcase, the end-user may still receive the benefits of location during anemergency, but at no other times. In an example of the third case, anemployer who is the real owner of a particular wireless transmitter canoverride an end-user action by an employee who is using the wirelesstransmitter as part of the job but who may not desire to be located. TheWireless Location System may query the wireless communications system,as described above, to obtain the mapping of the identity contained inthe wireless transmission to other identities.

[0224] The additions and deletions by the end-user are effected bydialed sequences of characters and digits and pressing the “SEND” orequivalent button on the wireless transmitter. These sequences may beoptionally chosen and made known by the operator of the WirelessLocation System. For example, one sequence may be “*55 SEND” to disablelocation processing. Other sequences are also possible. When theend-user can dialed this prescribed sequence, the wireless transmitterwill transmit the sequence over one of the prescribed control channelsof the wireless communications system. Since the Wireless LocationSystem independently detects and demodulates all reverse control channeltransmissions, the Wireless Location System can independently interpretthe prescribed dialed sequence and make the appropriate feature updatesto the Tasking List, as described above. When the Wireless LocationSystem has completed the update to the Tasking List, the WirelessLocation System commands the wireless communications system to send aconfirmation to the end-user. As described earlier, this may take theform of an audible tone, recorded or synthesized voice, or a textmessage. This command is executed over the interface between theWireless Location System and the wireless communications system.

[0225] Command Transmit—This command allows external applications tocause the Wireless Location System to send a command to the wirelesscommunications system to make a particular wireless transmitter, orgroup of wireless transmitters, transmit. This command may contain aflag or field that the wireless transmitter(s) should transmitimmediately or at a prescribed time. This command has the effort oflocating the wireless transmitter(s) upon command, since thetransmissions will be detected, demodulated, and triggered, causinglocation processing and the generation of a location record. This isuseful in eliminating or reducing any delay in determining location suchas waiting for the next registration time period for the wirelesstransmitter or waiting for an independent transmission to occur.

[0226] External Database Query and Update—The Wireless Location Systemincludes means to access an external database, to query the saidexternal database using the identity of the wireless transmitter orother parameters contained in the transmission or the trigger criteria,and to merge the data obtained from the external database with the datagenerated by the Wireless Location System to create a new enhancedlocation record. The enhanced location record may then be forwarded torequesting applications. The external database may contain, for example,data elements such as customer information, medical information,subscribed features, application related information, customer accountinformation, contact information, or sets of prescribed actions to takeupon a location trigger event. The Wireless Location System may alsocause updates to the external database, for example, to increment ordecrement a billing counter associated with the provision of locationservices, or to update the external database with the latest locationrecord associated with the particular wireless transmitter. The WirelessLocation System contains means to performed the actions described hereinon more than one external database. The list and sequence of externaldatabases to access and the subsequent actions to take are contained inone of the fields contained in the trigger criteria in the Tasking List.

[0227] Random Anonymous Location Processing—The Wireless Location Systemincludes means to perform large scale random anonymous locationprocessing. This function is valuable to certain types of applicationsthat require the gathering of a large volume of data about a populationof wireless transmitters without consideration to the specificidentities of the individual transmitters. Applications of this typeinclude: RF Optimization, which enables wireless carriers to measure theperformance of the wireless communications system by simultaneouslydetermining location and other parameters of a transmission; TrafficManagement, which enables government agencies and commercial concerns tomonitor the flow of traffic on various highways using statisticallysignificant samples of wireless transmitters travelling in vehicles; andLocal Traffic Estimation, which enables commercial enterprises toestimate the flow of traffic around a particular area which may helpdetermine the viability of particular businesses.

[0228] Applications requesting random anonymous location processingoptionally receive location records from two sources: (i) a copy oflocation records generated for other applications, and (ii) locationrecords which have been triggered randomly by the Wireless LocationSystem without regard to any specific criteria. All of the locationrecords generated from either source are forwarded with all of theidentity and trigger criteria information removed from the locationrecords; however, the requesting application(s) can determine whetherthe record was generated from the fully random process or is a copy fromanother trigger criteria. The random location records are generated by alow priority task within the Wireless Location System that performslocation processing on randomly selected transmissions wheneverprocessing and communications resources are available and wouldotherwise be unused at a particular instant in time. The requestingapplication(s) can specify whether the random location processing isperformed over the entire coverage area of a Wireless Location System,over specific geographic areas such as along prescribed highways, or bythe coverage areas of specific cell sites. Thus, the requestingapplication(s) can direct the resources of the Wireless Location Systemto those area of greatest interest to each application. Depending on therandomness desired by the application(s), the Wireless Location Systemcan adjust preferences for randomly selecting certain types oftransmissions, for example, registration messages, origination messages,page response messages, or voice channel transmissions.

[0229] Anonymous Tracking of a Geographic Group—The Wireless LocationSystem includes means to trigger location processing on a repetitivebasis for anonymous groups of wireless transmitters within a prescribedgeographic area. For example, a particular location application maydesire to monitor the travel route of a wireless transmitter over aprescribed period of time, but without the Wireless Location Systemdisclosing the particular identity of the wireless transmitter. Theperiod of time may be many hours, days, or weeks. Using the means, theWireless Location System: randomly selects a wireless transmitter thatinitiates a transmission in the geographic area of interest to theapplication; performs location processing on the transmission ofinterest; irreversibly translates and encrypts the identity of thewireless transmitter into a new coded identifier; creates a locationrecord using only the new coded identifier as an identifying means;forwards the location record to the requesting location application(s);and creates a dynamic task in the Tasking List for the wirelesstransmitter, wherein the dynamic task has an associated expiration time.Subsequently, whenever the prescribed wireless transmitter initiatestransmission, the Wireless Location System shall trigger using thedynamic task, perform location processing on the transmission ofinterest, irreversibly translate and encrypt the identity of thewireless transmitter into the new coded identifier using the same meansas prior such that the coded identifier is the same, create a locationrecord using the coded identifier, and forward the location record tothe requesting location application(s). The means described herein canbe combined with other functions of the Wireless Location System toperform this type of monitoring use either control or voice channeltransmissions. Further, the means described herein completely preservethe private identity of the wireless transmitter, yet enables anotherclass of applications that can monitor the travel patterns of wirelesstransmitters. This class of applications can be of great value indetermining the planning and design of new roads, alternate routeplanning, or the construction of commercial and retail space.

[0230] Location Record Grouping, Sorting, and Labeling—The WirelessLocation System include means to post-process the location records forcertain requesting applications to group, sort, or label the locationrecords. For each interface supported by the Wireless Location System,the Wireless Location System stores a profile of the types of data forwhich the application is both authorized and requesting, and the typesof filters or post-processing actions desired by the application. Manyapplications, such as the examples contained herein, do not requireindividual location records or the specific identities of individualtransmitters. For example, an RF optimization application derives morevalue from a large data set of location records for a particular cellsite or channel than it can from any individual location record. Foranother example, a traffic monitoring application requires only locationrecords from transmitters that are on prescribed roads or highways, andadditionally requires that these records be grouped by section of roador highway and by direction of travel. Other applications may requestthat the Wireless Location System forward location records that havebeen formatted to enhance visual display appeal by, for example,adjusting the location estimate of the transmitter so that thetransmitter's location appears on an electronic map directly on a drawnroad segment rather than adjacent to the road segment. Therefore, theWireless Location System preferably “snaps” the location estimate to thenearest drawn road segment.

[0231] The Wireless Location System can filter and report locationrecords to an application for wireless transmitters communicating onlyon a particular cell site, sector, RF channel, or group of RF channels.Before forwarding the record to the requesting application, the WirelessLocation System first verifies that the appropriate fields in the recordsatisfy the requirements. Records not matching the requirements are notforwarded, and records matching the requirements are forwarded. Somefilters are geographic and must be calculated by the Wireless LocationSystem. For example, the Wireless Location System can process a locationrecord to determine the closest road segment and direction of travel ofthe wireless transmitter on the road segment. The Wireless LocationSystem can then forward only records to the application that aredetermined to be on a particular road segment, and can further enhancethe location record by adding a field containing the determined roadsegment. In order to determine the closest road segment, the WirelessLocation System is provided with a database of road segments of interestby the requesting application. This database is stored in a table whereeach road segment is stored with a latitude and longitude coordinatedefining the end point of each segment. Each road segment can be modeledas a straight or curved line, and can be modeled to support one or twodirections of travel.

[0232] Then for each location record determined by the Wireless LocationSystem, the Wireless Location System compares the latitude and longitudein the location record to each road segment stored in the database, anddetermines the shortest distance from a modeled line connecting the endpoints of the segment to the latitude and longitude of the locationrecord. The shortest distance is a calculated imaginary line orthogonalto the line connecting the two end points of the stored road segment.When the closest road segment has been determined, the Wireless LocationSystem can further determine the direction of travel on the road segmentby comparing the direction of travel of the wireless transmitterreported by the location processing to the orientation of the roadsegment. The direction that produces the smallest error with respect tothe orientation of the road segments is then reported by the WirelessLocation System.

[0233] Network Operations Console (NOC) 16

[0234] The NOC 16 is a network management system that permits operatorsof the Wireless Location System easy access to the programmingparameters of the Wireless Location System. For example, in some cities,the Wireless Location System may contain many hundreds or even thousandsof SCS's 10. The NOC is the most effective way to manage a largeWireless Location System, using graphical user interface capabilities.The NOC will also receive real time alerts if certain functions withinthe Wireless Location System are not operating properly. These real timealerts can be used by the operator to take corrective action quickly andprevent a degradation of location service. Experience with trials of theWireless Location System show that the ability of the system to maintaingood location accuracy over time is directly related to the operator'sability to keep the system operating within its predeterminedparameters.

[0235] Location Processing

[0236] The Wireless Location System is capable of performing locationprocessing using two different methods known as central based processingand station based processing. Both techniques were first disclosed inU.S. Pat. No. 5,327,144, and are further enhanced in this specification.Location processing depends in part on the ability to accuratelydetermine certain phase characteristics of the signal as received atmultiple antennas and at multiple SCS's 10. Therefore, it is an objectof the Wireless Location System to identify and remove sources of phaseerror that impede the ability of the location processing to determinethe phase characteristics of the received signal. One source of phaseerror is inside of the wireless transmitter itself, namely theoscillator (typically a crystal oscillator) and the phase lock loopsthat allow the phone to tune to specific channels for transmitting.Lower cost crystal oscillators will generally have higher phase noise.Some air interface specifications, such as IS-136 and IS-95A, havespecifications covering the phase noise with which a wireless telephonecan transmit. Other air interface specifications, such as IS-553A, donot closely specify phase noise. It is therefore an object of thepresent invention to automatically reduce and/or eliminate a wirelesstransmitter's phase noise as a source of phase error in locationprocessing, in part by automatically selecting the use of central basedprocessing or station based processing. The automatic selection willalso consider the efficiency with which the communications link betweenthe SCS 10 and the TLP 12 is used, and the availability of DSP resourcesat each of the SCS 10 and TLP 12.

[0237] When using central based processing, the TDOA and FDOAdetermination and the multipath processing are performed in the TLP 12along with the position and speed determination. This method ispreferred when the wireless transmitter has a phase noise that is abovea predetermined threshold. In these cases, central based processing ismost effective in reducing or eliminating the phase noise of thewireless transmitter as a source of phase error because the TDOAestimate is performed using a digital representation of the actual RFtransmission from two antennas, which may be at the same SCS 10 ordifferent SCS's 10. In this method, those skilled in the art willrecognize that the phase noise of the transmitter is a common mode noisein the TDOA processing, and therefore is self-canceling in the TDOAdetermination process. This method works best, for example, with manyvery low cost AMPS cellular telephones that have a high phase noise. Thebasic steps in central based processing include the steps recited belowand represented in the flowchart of FIG. 6:

[0238] a wireless transmitter initiates a transmission on either acontrol channel or a voice channel (step S50);

[0239] the transmission is received at multiple antennas and at multipleSCS's 10 in the Wireless Location System (step S51);

[0240] the transmission is converted into a digital format in thereceiver connected to each SCS/antenna (step S52);

[0241] the digital data is stored in a memory in the receivers in eachSCS 10 (step S53);

[0242] the transmission is demodulated (step S54);

[0243] the Wireless Location System determines whether to begin locationprocessing for the transmission (step S55);

[0244] if triggered, the TLP 12 requests copies of the digital data fromthe memory in receivers at multiple SCS's 10 (step S56);

[0245] digital data is sent from multiple SCS's 10 to a selected TLP 12(step S57);

[0246] the TLP 12 performs TDOA, FDOA, and multipath mitigation on thedigital data from pairs of antennas (step S58);

[0247] the TLP 12 performs position and speed determination using theTDOA data, and then creates a location record and forwards the locationrecord to the AP 14 (step S59).

[0248] The Wireless Location System uses a variable number of bits torepresent the transmission when sending digital data from the SCS's 10to the TLP 12. As discussed earlier, the SCS receiver digitizes wirelesstransmissions with a high resolution, or a high number of bits perdigital sample in order to achieve a sufficient dynamic range. This isespecially required when using wideband digital receivers, which may besimultaneously receiving signals near to the SCS 10A and far from theSCS 10B. For example, up to 14 bits may be required to represent adynamic range of 84 dB. Location processing does not always require thehigh resolution per digital sample, however. Frequently, locations ofsufficient accuracy are achievable by the Wireless Location System usinga fewer number of bits per digital sample. Therefore, to minimize theimplementation cost of the Wireless Location System by conservingbandwidth on the communication links between each SCS 10 and TLP 12, theWireless Location System determines the fewest number of bits requiredto digitally represent a transmission while still maintaining a desiredaccuracy level. This determination is based, for example, on theparticular air interface protocol used by the wireless transmitter, theSNR of the transmission, the degree to which the transmission has beenperturbed by fading and/or multipath, and the current state of theprocessing and communication queues in each SCS 10. The number of bitssent from the SCS 10 to the TLP 12 are reduced in two ways: the numberof bits per sample is minimized, and the shortest length, or fewestsegments, of the transmission possible is used for location processing.The TLP 12 can use this minimal RF data to perform location processingand then compare the result with the desired accuracy level. Thiscomparison is performed on the basis of a confidence intervalcalculation. If the location estimate does not fall within the desiredaccuracy limits, the TLP 12 will recursively request additional datafrom selected SCS's 10. The additional data may include an additionalnumber of bits per digital sample and/or may include more segments ofthe transmission. This process of requesting additional data maycontinue recursively until the TLP 12 has achieved the prescribedlocation accuracy.

[0249] There are additional details to the basic steps described above.These details are described in prior U.S. Pat. Nos 5,327,144 and5,608,410 in other parts of this specification. One enhancement to theprocesses described in earlier patents is the selection of a singlereference SCS/antenna that is used for each baseline in the locationprocessing. In prior art, baselines were determined using pairs ofantenna sites around a ring. In the present Wireless Location System,the single reference SCS/antenna used is generally the highest SNRsignal, although other criteria are also used as described below. Theuse of a high SNR reference aids central based location processing whenthe other SCS/antennas used in the location processing are very weak,such as at or below the noise floor (i.e. zero or negative signal tonoise ratio). When station based location processing is used, thereference signal is a re-modulated signal, which is intentionallycreated to have a very high signal to noise ratio, further aidinglocation processing for very weak signals at other SCS/antennas. Theactual selection of the reference SCS/antenna is described below.

[0250] The Wireless Location System mitigates multipath by firstrecursively estimating the components of multipath received in additionto the direct path component and then subtracting these components fromthe received signal. Thus the Wireless Location System models thereceived signal and compares the model to the actual received signal andattempts to minimize the difference between the two using a weightedleast square difference. For each transmitted signal x(t) from awireless transmitter, the received signal y(t) at each SCS/antenna is acomplex combination of signals:

y(t)=Σ×(t−τ _(n))a _(n) e ^(jω(t−τn)), for all n=0 to N;

[0251] where

[0252] x(t) is the signal as transmitted by the wireless transmitter;

[0253] a_(n) and τ_(n) are the complex amplitude and delays of themultipath components;

[0254] N is the total number of multipath components in the receivedsignal; and

[0255] a₀ and τ₀ are constants for the most direct path component.

[0256] The operator of the Wireless Location System empiricallydetermines a set of constraints for each component of multipath thatapplies to the specific environment in which each Wireless LocationSystem is operating. The purpose of the constraints is to limit theamount of processing time that the Wireless Location System spendsoptimizing the results for each multipath mitigation calculation. Forexample, the Wireless Location System may be set to determine only fourcomponents of multipath: the first component may be assumed to have atime delay in the range τ_(1A) to τ_(1B); the second component may beassumed to have a time delay in the range τ_(2A) to τ_(2B); the thirdcomponent may be assumed to have a time delay in the range τ_(3A) toτ_(3B); and similar for the fourth component; however the fourthcomponent is a single value that effectively represents a complexcombination of many tens of individual (and somewhat diffuse) multipathcomponents whose time delays exceed the range of the third component.For ease of processing, the Wireless Location System transforms theprior equation into the frequency domain, and then solves for theindividual components such that a weighted least squares difference isminimized.

[0257] When using station based processing, the TDOA and FDOAdetermination and multipath mitigation are performed in the SCS's 10,while the position and speed determination are typically performed inthe TLP 12. The main advantage of station based processing, as describedin U.S. Pat. No. 5,327,144, is reducing the amount of data that is senton the communication link between each SCS 10 and TLP 12. However, theremay be other advantages as well. One new objective of the presentinvention is increasing the effective signal processing gain during theTDOA processing. As pointed out earlier, central based processing hasthe advantage of eliminating or reducing phase error caused by the phasenoise in the wireless transmitter. However, no previous disclosure hasaddressed how to eliminate or reduce the same phase noise error whenusing station based processing. The present invention reduces the phaseerror and increases the effective signal processing gain using the stepsrecited below and shown in FIG. 6:

[0258] a wireless transmitter initiates a transmission on either acontrol channel or a voice channel (step S60);

[0259] the transmission is received at multiple antennas and at multipleSCS's 10 in the Wireless Location System (step S61);

[0260] the transmission is converted into a digital format in thereceiver connected to each antenna (step S62);

[0261] the digital data is stored in a memory in the SCS 10 (step S63);

[0262] the transmission is demodulated (step S64);

[0263] the Wireless Location System determines whether to begin locationprocessing for the transmission (step S65);

[0264] if triggered, a first SCS 10A demodulates the transmission anddetermines an appropriate phase correction interval (step S66);

[0265] for each such phase correction interval, the first SCS 10Acalculates an appropriate phase correction and amplitude correction, andencodes this phase correction parameter and amplitude correctionparameter along with the demodulated data (step S67);

[0266] the demodulated data and phase correction and amplitudecorrection parameters are sent from the first SCS 10A to a TLP 12 (stepS68);

[0267] the TLP 12 determines the SCS's 10 and receiving antennas to usein the location processing (step S69);

[0268] the TLP 12 sends the demodulated data and phase correction andamplitude correction parameters to each second SCS 10B that will be usedin the location processing (step S70);

[0269] the first SCS 10 and each second SCS 10B creates a firstre-modulated signal based upon the demodulated data and the phasecorrection and amplitude correction parameters (step S71);

[0270] the first SCS 10A and each second SCS 10B performs TDOA, FDOA,and multipath mitigation using the digital data stored in memory in eachSCS 10 and the first re-modulated signal (step S72);

[0271] the TDOA, FDOA, and multipath mitigation data are sent from thefirst SCS 10A and each second SCS 10B to the TLP 12 (step S73);

[0272] the TLP 12 performs position and speed determination using theTDOA data (step S74); and

[0273] the TLP 12 creates a location record, and forwards the locationrecord to the AP 14 (step S75).

[0274] The advantages of determining phase correction and amplitudecorrection parameters are most obvious in the location of CDMA wirelesstransmitters based upon IS-95A. As is well known, the reversetransmissions from an IS-95A transmitter are sent using non-coherentmodulation. Most CDMA base stations only integrate over a single bitinterval because of the non-coherent modulation. For a CDMA AccessChannel, with a bit rate of 4800 bits per second, there are 256 chipssent per bit, which permits an integration gain of 24 dB. Using thetechnique described above, the TDOA processing in each SCS 10 mayintegrate, for example, over a full 160 millisecond burst (196,608chips) to produce an integration gain of 53 dB. This additionalprocessing gain enables the present invention to detect and locate CDMAtransmissions using multiple SCS's 10, even if the base stationscollocated with the SCS's 10 cannot detect the same CDMA transmission.

[0275] For a particular transmission, if either the phase correctionparameters or the amplitude correction parameters are calculated to bezero, or are not needed, then these parameters are not sent in order toconserve on the number of bits transmitted on the communications linkbetween each SCS 10 and TLP 12. In another embodiment of the invention,the Wireless Location System may use a fixed phase correction intervalfor a particular transmission or for all transmissions of a particularair interface protocol, or for all transmissions made by a particulartype of wireless transmitter. This may, for example, be based uponempirical data gathered over some period of time by the WirelessLocation System showing a reasonable consistency in the phase noiseexhibited by various classes of transmitters. In these cases, the SCS 10may save the processing step of determining the appropriate phasecorrection interval.

[0276] Those skilled in the art will recognize that there are many waysof measuring the phase noise of a wireless transmitter. In oneembodiment, a pure, noiseless re-modulated copy of the signal receivedat the first SCS 10A may be digitally generated by DSP's in the SCS,then the received signal may be compared against the pure signal overeach phase correction interval and the phase difference may be measureddirectly. In this embodiment, the phase correction parameter will becalculated as the negative of the phase difference over that phasecorrection interval. The number of bits required to represent the phasecorrection parameter will vary with the magnitude of the phasecorrection parameter, and the number of bits may vary for each phasecorrection interval. It has been observed that some transmissions, forexample, exhibit greater phase noise early in the transmission, and lessphase noise in the middle of and later in the transmission.

[0277] Station based processing is most useful for wireless transmittersthat have relatively low phase noise. Although not necessarily requiredby their respective air interface standards, wireless telephones thatuse the TDMA, CDMA, or GSM protocols will typically exhibit lower phasenoise. As the phase noise of a wireless transmitter increases, thelength of a phase correction interval may decrease and/or the number ofbits required to represent the phase correction parameters increases.Station based processing is not effective when the number of bitsrequired to represent the demodulated data plus the phase correction andamplitude parameters exceeds a predetermined proportion of the number ofbits required to perform central based processing. It is therefore anobject of the present invention to automatically determine for eachtransmission for which a location is desired whether to process thelocation using central based processing or station based processing. Thesteps in making this determination are recited below and shown in FIG.7:

[0278] a wireless transmitter initiates a transmission on either acontrol channel or a voice channel (step S80);

[0279] the transmission is received at a first SCS 10A (step S81);

[0280] the transmission is converted into a digital format in thereceiver connected to each antenna (step S82);

[0281] the Wireless Location System determines whether to begin locationprocessing for the transmission (step S83);

[0282] if triggered, a first SCS 10A demodulates the transmission andestimates an appropriate phase correction interval and the number ofbits required to encode the phase correction and amplitude correctionparameters (step S84);

[0283] the first SCS 10A then estimates the number of bits required forcentral based processing;

[0284] based upon the number of bits required for each respectivemethod, the SCS 10 or the TLP 12 determine whether to use central basedprocessing or station based processing to perform the locationprocessing for this transmission (step S85).

[0285] In another embodiment of the invention, the Wireless LocationSystem may always use central based processing or station basedprocessing for all transmissions of a particular air interface protocol,or for all transmissions made by a particular kind of wirelesstransmitter. This may, for example, be based upon empirical datagathered over some period of time by the Wireless Location Systemshowing a reasonable consistency in the phase noise exhibited by variousclasses of transmitters. In these cases, the SCS 10 and/or the TLP 12may be saved the processing step of determining the appropriateprocessing method.

[0286] A further enhancement of the present invention, used for bothcentral based processing and station based processing, is the use ofthreshold criteria for including baselines in the final determination oflocation and velocity of the wireless transmitter. For each baseline,the Wireless Location System calculates a number of parameters thatinclude: the SCS/antenna port used with the reference SCS/antenna incalculating the baseline, the peak, average, and variance in the powerof the transmission as received at the SCS/antenna port used in thebaseline and over the interval used for location processing, thecorrelation value from the cross-spectra correlation between theSCS/antenna used in the baseline and the reference SCS/antenna, thedelay value for the baseline, the multipath mitigation parameters, theresidual values remaining after the multipath mitigation calculations,the contribution of the SCS/antenna to the weighted GDOP in the finallocation solution, and a measure of the quality of fit of the baselineif included in the final location solution. Each baseline is included inthe final location solution is each meets or exceeds the thresholdcriteria for each of the parameters described herein. A baseline may beexcluded from the location solution if it fails to meet one or more ofthe threshold criteria. Therefore, it is frequently possible that thenumber of SCS/antennas actually used in the final location solution isless than the total number considered.

[0287] Previous U.S. Pat. Nos. 5,327,144 and 5,608,410 disclosed amethod by which the location processing minimized the least squaredifference (LSD) value of the following equation:

LSD=[Q ₁₂(Delay_(—) T ₁₂−Delay_(—) O ₁₂)² +Q ₁₃(Delay_(—) T ₁₃−Delay_(—)O ₁₃)² + . . . +Q _(xy)(Delay_(—) T _(xy)−Delay_(—) O _(xy))²

[0288] In the present implementation, this equation has been rearrangedto the following form in order to make the location processing code moreefficient:

LSD=Σ(TDOA _(0i)−τ_(i)+τ₀)² w _(i) ²; over all i=1 to N−1

[0289] where

[0290] N=number of SCS/antennas used in the location processing;

[0291] TDOA_(0i)=the TDOA to the i^(th) site from reference site 0;

[0292] τ_(i)=the theoretical line of sight propagation time from thewireless transmitter to the i^(th) site;

[0293] τ₀=the theoretical line of sight propagation time from thetransmitter to the reference; and

[0294] w_(i)=the weight, or quality factor, applied to the i^(th)baseline.

[0295] In the present implementation, the Wireless Location System alsouses another alternate form of the equation that can aid in determininglocation solutions when the reference signal is not very strong or whenit is likely that a bias would exist in the location solution using theprior form of the equation:

LSD′=Σ(TDOA _(0i)−τ_(i))² w _(i) ² −b ² Σw _(i) ²: over all i=0 to N−1

[0296] Where

[0297] N=number of SCS/antennas used in the location processing;

[0298] TDOA_(0i)=the TDOA to the i^(th) site from reference site 0;

[0299] TDOA₀₀=is assumed to be zero;

[0300] τ_(i)=the theoretical line of sight propagation time from thewireless transmitter to the i^(th) site;

[0301] b=a bias that is separately calculated for each theoretical pointthat minimizes LSD′ at that theoretical point; and

[0302] w_(i)=the weight, or quality factor, applied to the i^(th)baseline.

[0303] The LSD′ form of the equation offers an easier means of removinga bias in location solutions at the reference site by making w₀ equal tothe maximum value of the other weights or basing w₀ on the relativesignal strength at the reference site. Note that if w₀ is much largerthan the other weights, then b is approximately equal to τ₀. In general,the weights, or quality factors are based on similar criteria to thatdiscussed above for the threshold criteria in including baselines. Thatis, the results of the criteria calculations are used for weights andwhen the criteria falls below threshold the weight is then set to zeroand is effectively not included in the determination of the finallocation solution.

[0304] Antenna Selection Process for Location Processing

[0305] Previous inventions and disclosures, such as those listed above,have described techniques in which a first, second, or possibly thirdantenna site, cell site, or base station are required to determinelocation. U.S. Pat. No. 5,608,410 further discloses a Dynamic SelectionSubsystem (DSS) that is responsible for determining which data framesfrom which antenna site locations will be used to calculate the locationof a responsive transmitter. In the DSS, if data frames are receivedfrom more than a threshold number of sites, the DSS determines which arecandidates for retention or exclusion, and then dynamically organizesdata frames for location processing. The DSS prefers to use more thanthe minimum number of antenna sites so that the solution isover-determined. Additionally, the DSS assures that all transmissionsused in the location processing were received from the same transmitterand from the same transmission.

[0306] The preferred embodiments of the prior inventions had severallimitations, however. First, either only one antenna per antenna site(or cell site) is used, or the data from two or four diversity antennaswere first combined at the antenna site (or cell site) prior totransmission to the central site. Additionally, all antenna sites thatreceived the transmission sent data frames to the central site, even ifthe DSS later discarded the data frames. Thus, some communicationsbandwidth may have been wasted sending data that was not used.

[0307] The present inventors have determined that while a minimum of twoor three sites are required in order determine location, the actualselection of antennas and SCS's 10 to use in location processing canhave a significant effect on the results of the location processing. Inaddition, it is advantageous to include the means to use more than oneantenna at each SCS 10 in the location processing. The reason for usingdata from multiple antennas at a cell site independently in the locationprocessing is that the signal received at each antenna is uniquelyaffected by multipath, fading, and other disturbances. It is well knownin the field that when two antennas are separated in distance by morethan one wavelength, then each antenna will receive the signal on anindependent path. Therefore, there is frequently additional and uniqueinformation to be gained about the location of the wireless transmitterby using multiple antennas, and the ability of the Wireless LocationSystem to mitigate multipath is enhanced accordingly.

[0308] It is therefore an object of the present invention to provide animproved method for using the signals received from more than oneantenna at an SCS 10 in the location processing. It is a further objectto provide a method to improve the dynamic process used to select thecooperating antennas and SCS's 10 used in the location processing. Thefirst object is achieved by providing means within the SCS 10 to selectand use any segment of data collected from any number of antennas at anSCS in the location processing. As described earlier, each antenna at acell site is connected to a receiver internal to the SCS 10. Eachreceiver converts signals received from the antenna into a digital form,and then stores the digitized signals temporarily in a memory in thereceiver. The TLP 12 has been provided with means to direct any SCS 10to retrieve segments of data from the temporary memory of any receiver,and to provide the data for use in location processing. The secondobject is achieved by providing means within the Wireless LocationSystem to monitor a large number of antennas for reception of thetransmission that the Wireless Location System desires to locate, andthen selecting a smaller set of antennas for use in location processingbased upon a predetermined set of parameters. One example of thisselection process is represented by the flowchart of FIG. 8:

[0309] a wireless transmitter initiates a transmission on either acontrol channel or a voice channel (step S90);

[0310] the transmission is received at multiple antennas and at multipleSCS's 10 in the Wireless Location System (step S91);

[0311] the transmission is converted into a digital format in thereceiver connected to each antenna (step S92);

[0312] the digital data is stored in a memory in each SCS 10 (step S93);

[0313] the transmission is demodulated at at least one SCS 10A and thechannel number on which the transmission occurred and the cell site andsector serving the wireless transmitter is determined (step S94);

[0314] based upon the serving cell site and sector, one SCS 10A isdesignated as the ‘primary’ SCS 10 for processing that transmission(step S95);

[0315] the primary SCS 10A determines a timestamp associated with thedemodulated data (step S96);

[0316] the Wireless Location System determines whether to begin locationprocessing for the transmission (step S97);

[0317] if location processing is triggered, the Wireless Location Systemdetermines a candidate list of SCS's 10 and antennas to use in thelocation processing (step S98);

[0318] each candidate SCS/antenna measures and reports severalparameters in the channel number of the transmission and at the time ofthe timestamp determined by the primary SCS 10A (step S99);

[0319] the Wireless Location System orders the candidate SCS/antennasusing specified criteria and selects a reference SCS/antenna and aprocessing list of SCS/antennas to use in the location processing (stepS100); and

[0320] the Wireless Location System proceeds with location processing asdescribed earlier, using data from the processing list of SCS/antennas(step S101).

[0321] Selecting Primary SCS/Antenna

[0322] The process for choosing the ‘primary’ SCS/antenna is critical,because the candidate list of SCS's 10 and antennas 10-1 is determinedin part based upon the designation of the primary SCS/antenna. When awireless transmitter makes a transmission on a particular RF channel,the transmission frequently can propagate many miles before the signalattenuates below a level at which it can be demodulated. Therefore,there are frequently many SCS/antennas capable of demodulating thesignal. This especially occurs is urban and suburban areas where thefrequency re-use pattern of many wireless communications systems can bequite dense. For example, because of the high usage rate of wireless andthe dense cell site spacing, the present inventors have tested wirelesscommunications systems in which the same RF control channel and digitalcolor code were used on cell sites spaced about one mile apart. Becausethe Wireless Location System is independently demodulating thesetransmissions, the Wireless Location System frequently can demodulatethe same transmission at two, three, or more separate SCS/antennas. TheWireless Location System detects that the same transmission has beendemodulated multiple times at multiple SCS/antennas when the WirelessLocation System receives multiple demodulated data frames sent fromdifferent SCS/antennas, each with a number of bit errors below apredetermined bit error threshold, and with the demodulated datamatching within an acceptable limit of bit errors, and all occurringwithin a predetermined interval of time.

[0323] When the Wireless Location System detects demodulated data frommultiple SCS/antennas, it examines the following parameters to determinewhich SCS/antenna shall be designated the primary SCS: average SNR overthe transmission interval used for location processing, the variance inthe SNR over the same interval, correlation of the beginning of thereceived transmission against a pure pre-cursor (i.e. for AMPS, thedotting and Barker code), the number of bit errors in the demodulateddata, and the magnitude and rate of change of the SNR from just beforethe on-set of the transmission to the on-set of the transmission, aswell as other similar parameters. The average SNR is typicallydetermined at each SCS/antenna either over the entire length of thetransmission to be used for location processing, or over a shorterinterval. The average SNR over the shorter interval can be determined byperforming a correlation with the dotting sequence and/or Barker codeand/or sync word, depending on the particular air interface protocol,and over a short range of time before, during, and after the timestampreported by each SCS 10. The time range may typically be +/−200microseconds centered at the timestamp, for example. The WirelessLocation System will generally order the SCS/antennas using thefollowing criteria, each of which may be weighted (multiplied by anappropriate factor) when combining the criteria to determine the finaldecision: SCS/antennas with a lower number of bit errors are preferredto SCS/antennas with a higher number of bit errors, average SNR for agiven SCS/antenna must be greater than a predetermined threshold to bedesignated as the primary; SCS/antennas with higher average SNR arepreferred over those with lower average SNR; SCS/antennas with lower SNRvariance are preferred to those with higher SNR variance; andSCS/antennas with a faster SNR rate of change at the on-set of thetransmission are preferred to those with a slower rate of change. Theweighting applied to each of these criteria may be adjusted by theoperator of the Wireless Location System to suit the particular designof each system.

[0324] The candidate list of SCS's 10 and antennas 10-1 are selectedusing a predetermined set of criteria based, for example, upon knowledgeof the types of cell sites, types of antennas at the cell sites,geometry of the antennas, and a weighting factor that weights certainantennas more than other antennas. The weighting factor takes intoaccount knowledge of the terrain in which the Wireless Location Systemis operating, past empirical data on the contribution of each antennahas made to good location estimates, and other factors that may bespecific to each different WLS installation. In one embodiment, forexample, the Wireless Location System may select the candidate list toinclude all SCS's 10 up to a maximum number of sites(max_number_of_sites) that are closer than a predefined maximum radiusfrom the primary site (max_radius_from_primary). For example, in anurban or suburban environment, wherein there may be a large number ofcell sites, the max_number_of_sites may be limited to nineteen. Nineteensites would include the primary, the first ring of six sites surroundingthe primary (assuming a classic hexagonal distribution of cell sites),and the next ring of twelve sites surrounding the first ring. This isdepicted in FIG. 9. In another embodiment, in a suburban or ruralenvironment, max_radius_from_primary may be set to 40 miles to ensurethat the widest possible set of candidate SCS/antennas is available. TheWireless Location System is provided with means to limit the totalnumber of candidate SCS's 10 to a maximum number(max_number_candidates), although each candidate SCS may be permitted tochoose the best port from among its available antennas. This limits themaximum time spent by the Wireless Location System processing aparticular location. Max_number_candidates may be set to thirty-two, forexample, which means that in a typical three sector wirelesscommunications system with diversity, up to 32*6=192 total antennascould be considered for location processing for a particulartransmission. In order to limit the time spent processing a particularlocation, the Wireless Location System is provided with means to limitthe number of antennas used in the location processing tomax_number_antennas_processed. Max_number_antennas_processed isgenerally less than max_number_candidates, and is typically set tosixteen.

[0325] While the Wireless Location System is provided with the abilityto dynamically determine the candidate list of SCS's 10 and antennasbased upon the predetermined set of criteria described above, theWireless Location System can also store a fixed candidate list in atable. Thus, for each cell site and sector in the wirelesscommunications system, the Wireless Location System has a separate tablethat defines the candidate list of SCS's 10 and antennas 10-1 to usewhenever a wireless transmitter initiates a transmission in that cellsite and sector. Rather than dynamically choose the candidateSCS/antennas each time a location request is triggered, the WirelessLocation System reads the candidate list directly from the table whenlocation processing is initiated.

[0326] In general, a large number of candidate SCS's 10 is chosen toprovide the Wireless Location System with sufficient opportunity andability to measure and mitigate multipath. On any given transmission,any one or more particular antennas at one or more SCS's 10 may receivesignals that have been affected to varying degrees by multipath.Therefore, it is advantageous to provide this means within the WirelessLocation System to dynamically select a set of antennas which may havereceived less multipath than other antennas. The Wireless LocationSystem uses various techniques to mitigate as much multipath as possiblefrom any received signal; however it is frequently prudent to choose aset of antennas that contain the least amount of multipath.

[0327] Choosing Reference and Cooperating SCS/Antennas

[0328] In choosing the set of SCS/antennas to use in locationprocessing, the Wireless Location System orders the candidateSCS/antennas using several criteria, including for example: average SNRover the transmission interval used for location processing, thevariance in the SNR over the same interval, correlation of the beginningof the received transmission against a pure pre-cursor (i.e. for AMPS,the dotting and Barker code) and/or demodulated data from the primarySCS/antenna, the time of the on-set of the transmission relative to theon-set reported at the SCS/antenna at which the transmission wasdemodulated, and the magnitude and rate of change of the SNR from justbefore the on-set of the transmission to the on-set of the transmission,as well as other similar parameters. The average SNR is typicallydetermined at each SCS, and for each antenna in the candidate listeither over the entire length of the transmission to be used forlocation processing, or over a shorter interval. The average SNR overthe shorter interval can be determined by performing a correlation withthe dotting sequence and/or Barker code and/or sync word, depending onthe particular air interface protocol, and over a short range of timebefore, during, and after the timestamp reported by the primary SCS 10.The time range may typically be +/−200 microseconds centered at thetimestamp, for example. The Wireless Location System will generallyorder the candidate SCS/antennas using the following criteria, each ofwhich may be weighted when combining the criteria to determine the finaldecision: average SNR for a given SCS/antenna must be greater than apredetermined threshold to be used in location processing; SCS/antennaswith higher average SNR are preferred over those with lower average SNR;SCS/antennas with lower SNR variance are preferred to those with higherSNR variance; SCS/antennas with an on-set closer to the on-set reportedby the demodulating SCS/antenna are preferred to those with an on-setmore distant in time; SCS/antennas with a faster SNR rate of change arepreferred to those with a slower rate of change; SCS/antennas with lowerincremental weighted GDOP are preferred over those with higherincremental weighted GDOP, wherein the weighting is based upon estimatedpath loss from the primary SCS. The weighting applied to each of thesepreferences may be adjusted by the operator of the Wireless LocationSystem to suit the particular design of each system. The number ofdifferent SCS's 10 used in the location processing is maximized up to apredetermined limit; the number of antennas used at each SCS 10 inlimited to a predetermined limit; and the total number of SCS/antennasused is limited to max_number_antennas_processed. The SCS/antenna withthe highest ranking using the above described process is designated asthe reference SCS/antenna for location processing.

[0329] Best Port Selection within an SCS 10

[0330] Frequently, the SCS/antennas in the candidate list or in the listto use in location processing will include only one or two antennas at aparticular SCS 10. In these cases, the Wireless Location System maypermit the SCS 10 to choose the “best port” from all or some of theantennas at the particular SCS 10. For example, if the Wireless LocationSystem chooses to use only one antenna at a first SCS 10, then the firstSCS 10 may select the best antenna port from the typical six antennaports that are connected to that SCS 10, or it may choose the bestantenna port from among the two antenna ports of just one sector of thecell site. The best antenna port is chosen by using the same process andcomparing the same parameters as described above for choosing the set ofSCS/antennas to use in location processing, except that all of theantennas being considered for best port are all in the same SCS 10. Incomparing antennas for best port, the SCS 10 may also optionally dividethe received signal into segments, and then measure the SNR separatelyin each segment of the received signal. Then, the SCS 10 can optionallychoose the best antenna port with highest SNR either by (i) using theantenna port with the most segments with the highest SNR, (ii) averagingthe SNR in all segments and using the antenna port with the highestaverage SNR, or (iii) using the antenna port with the highest SNR in anyone segment.

[0331] Detection and Recovery from Collisions

[0332] Because the Wireless Location System will use data from manySCS/antenna ports in location processing, there is a chance that thereceived signal at one or more particular SCS/antenna ports containsenergy that is co-channel interference from another wireless transmitter(i.e. a partial or full collision between two separate wirelesstransmissions has occurred). There is also a reasonable probability thatthe co-channel interference has a much higher SNR than the signal fromthe target wireless transmitter, and if not detected by the WirelessLocation System, the co-channel interference may cause an incorrectchoice of best antenna port at an SCS 10, reference SCS/antenna,candidate SCS/antenna, or SCS/antenna to be used in location processing.The co-channel interference may also cause poor TDOA and FDOA results,leading to a failed or poor location estimate. The probability ofcollision increases with the density of cell sites in the host wirelesscommunications system, especially in dense suburban or ruralenvironments where the frequencies are re-used often and wireless usageby subscribers is high.

[0333] Therefore, the Wireless Location System includes means to detectand recover from the types of collisions described above. For example,in the process of selecting a best port, reference SCS/antenna, orcandidate SCS/antenna, the Wireless Location System determines theaverage SNR of the received signal and the variance of the SNR over theinterval of the transmission; when the variance of the SNR is above apredetermined threshold, the Wireless Location System assigns aprobability that a collision has occurred. If the signal received at anSCS/antenna has increased or decreased its SNR in a single step, and byan amount greater than a predetermined threshold, the Wireless LocationSystem assigns a probability that a collision has occurred. Further, ifthe average SNR of the signal received at a remote SCS is greater thanthe average SNR that would be predicted by a propagation model, giventhe cell site at which the wireless transmitter initiated itstransmission and the known transmit power levels and antenna patterns ofthe transmitter and receive antennas, the Wireless Location Systemassigns a probability that a collision has occurred. If the probabilitythat a collision has occurred is above a predetermined threshold, thenthe Wireless Location System performs the further processing describedbelow to verify whether and to what extent a collision may have impairedthe received signal at an SCS/antenna. The advantage of assigningprobabilities is to reduce or eliminate extra processing for themajority of transmissions for which collisions have not occurred. Itshould be noted that the threshold levels, assigned probabilities, andother details of the collision detection and recovery processesdescribed herein are configurable, i.e., selected based on theparticular application, environment, system variables, etc., that wouldaffect their selection.

[0334] For received transmissions at an SCS/antenna for which theprobability of a collision is above the predetermined threshold andbefore using RF data from a particular antenna port in a referenceSCS/antenna determination, best port determination or in locationprocessing, the Wireless Location System preferably verifies that the RFdata from each antenna port is from the correct wireless transmitter.This is determined, for example, by demodulating segments of thereceived signal to verify, for example, that the MIN, MSID, or otheridentifying information is correct or that the dialed digits or othermessage characteristics match those received by the SCS/antenna thatinitially demodulated the transmission. The Wireless Location System mayalso correlate a short segment of the received signal at an antenna portwith the signal received at the primary SCS 10 to verify that thecorrelation result is above a predetermined threshold. If the WirelessLocation System detects that the variance in the SNR over the entirelength of the transmission is above a pre-determined threshold, theWireless Location System may divide the transmission into segments andtest each segment as described herein to determine whether the energy inthat segment is primarily from the signal from the wireless transmitterfor which location processing has been selected or from an interferingtransmitter.

[0335] The Wireless Location System may choose to use the RF data from aparticular SCS/antenna in location processing even if the WirelessLocation System has detected that a partial collision has occurred atthat SCS/antenna. In these cases, the SCS 10 uses the means describedabove to identify that portion of the received transmission whichrepresents a signal from the wireless transmitter for which locationprocessing has been selected, and that portion of the receivedtransmission which contains co-channel interference. The WirelessLocation System may command the SCS 10 to send or use only selectedsegments of the received transmission that do not contain the co-channelinterference. When determining the TDOA and FDOA for a baseline usingonly selected segments from an SCS/antenna, the Wireless Location Systemuses only the corresponding segments of the transmission as received atthe reference SCS/antenna. The Wireless Location System may continue touse all segments for baselines in which no collisions were detected. Inmany cases, the Wireless Location System is able to complete locationprocessing and achieve an acceptable location error using only a portionof the transmission. This inventive ability to select the appropriatesubset of the received transmission and perform location processing on asegment by segment basis enables the Wireless Location System tosuccessfully complete location processing in cases that might havefailed using previous techniques.

[0336] Multiple Pass Location Processing

[0337] Certain applications may require a very fast estimate of thegeneral location of a wireless transmitter, followed by a more accurateestimate of the location that can be sent subsequently. This can bevaluable, for example, for E9-1-1 systems that handle wireless calls andmust make a call routing decision very quickly, but can wait a littlelonger for a more exact location to be displayed upon the E9-1-1call-taker's electronic map terminal. The Wireless Location Systemsupports these applications with an inventive multiple pass locationprocessing mode.

[0338] In many cases, location accuracy is enhanced by using longersegments of the transmission and increasing the processing gain throughlonger integration intervals. But longer segments of the transmissionrequire longer processing periods in the SCS 10 and TLP 12, as well aslonger time periods for transmitting the RF data across thecommunications interface from the SCS 10 to the TLP 12. Therefore, theWireless Location System includes means to identify those transmissionsthat require a fast but rough estimate of the location followed by morecomplete location processing that produces a better location estimate.The Signal of Interest Table includes a flag for each Signal of Interestthat requires a multiple pass location approach. This flag specifies themaximum amount of time permitted by the requesting location applicationfor the first estimate to be sent, as well as the maximum amount of timepermitted by the requesting location application for the final locationestimate to be sent. The Wireless Location System performs the roughlocation estimate by selecting a subset of the transmission for which toperform location processing. the Wireless Location System may choose,for example, the segment that was identified at the primary SCS/antennawith the highest average SNR. After the rough location estimate has beendetermined, using the methods described earlier, but with only a subsetof the transmission, the TLP 12 forwards the location estimate to the AP14, which then forwards the rough estimate to the requesting applicationwith a flag indicating that the estimate is only rough. The WirelessLocation System then performs its standard location processing using allof the aforementioned methods, and forwards this location estimate witha flag indicating the final status of this location estimate. TheWireless Location System may perform the rough location estimate and thefinal location estimate sequentially on the same DSP in a TLP 12, or mayperform the location processing in parallel on different DSP's. Parallelprocessing may be necessary to meet the maximum time requirements of therequesting location applications. The Wireless Location System supportsdifferent maximum time requirements from different location applicationsfor the same wireless transmission.

[0339] Very Short Baseline TDOA

[0340] The Wireless Location System is designed to operate in urban,suburban, and rural areas. In rural areas, when there are not sufficientcell sites available from a single wireless carrier, the WirelessLocation System can be deployed with SCS's 10 located at the cell sitesof other wireless carriers or at other types of towers, including AM orFM radio station, paging, and two-way wireless towers. In these cases,rather than sharing the existing antennas of the wireless carrier, theWireless Location System may require the installation of appropriateantennas, filters, and low noise amplifiers to match the frequency bandof the wireless transmitters of interest to be located. For example, anAM radio station tower may require the addition of 800 MHz antennas tolocate cellular band transmitters. There may be cases, however, whereinno additional towers of any type are available at reasonable cost andthe Wireless Location System must be deployed on just a few towers ofthe wireless carrier. In these cases, the Wireless Location Systemsupports an antenna mode known as very short baseline TDOA. This antennamode becomes active when additional antennas are installed on a singlecell site tower, whereby the antennas are placed at a distance of lessthan one wavelength apart. This may require the addition of just oneantenna per cell site sector such that the Wireless Location System usesone existing receive antenna in a sector and one additional antenna thathas been placed next to the existing receive antenna. Typically, the twoantennas in the sector are oriented such that the primary axes, or lineof direction, of the main beams are parallel and the spacing between thetwo antenna elements is known with precision. In addition, the two RFpaths from the antenna elements to the receivers in the SCS 10 arecalibrated.

[0341] In its normal mode, the Wireless Location System determines theTDOA and FDOA for pairs of antenna that are separated by manywavelengths. For a TDOA on a baseline using antennas from two differencecell sites, the pairs of antennas are separated by thousands ofwavelengths. For a TDOA on a baseline using antennas at the same cellsite, the pairs of antennas are separated by tens of wavelengths. Ineither case, the TDOA determination effectively results in a hyperbolicline bisecting the baseline and passing through the location of thewireless transmitter. When antennas are separated by multiplewavelengths, the received signal has taken independent paths from thewireless transmitter to each antenna, including experiencing differentmultipath and Doppler shifts. However, when two antennas are closer thanone wavelength, the two received signals have taken essentially the samepath and experienced the same fading, multipath, and Doppler shift.Therefore, the TDOA and FDOA processing of the Wireless Location Systemtypically produces a Doppler shift of zero (or near-zero) hertz, and atime difference on the order of zero to one nanosecond. A timedifference that short is equivalent to an unambiguous phase differencebetween the signals received at the two antennas on the very shortbaseline. For example, at 834 MHz, the wavelength of an AMPS reversecontrol channel transmission is about 1.18 feet. A time difference of0.1 nanoseconds is equivalent to a received phase difference of about 30degrees. In this case, the TDOA measurement produces a hyperbola that isessentially a straight line, still passing through the location of thewireless transmitter, and in a direction that is rotated 30 degrees fromthe direction of the parallel lines formed by the two antennas on thevery short baseline. When the results of this very short baseline TDOAat the single cell site are combined with a TDOA measurement on abaseline between two cell sites, the Wireless Location System candetermine a location estimate using only two cell sites.

[0342] Bandwidth Monitoring Method for Improving Location Accuracy

[0343] AMPS cellular transmitters presently comprise the large majorityof the wireless transmitters used in the U.S. and AMPS reverse voicechannel transmissions are generally FM signals modulated by both voiceand a supervisory audio tone (SAT). The voice modulation is standard FM,and is directly proportional to the speaking voice of the person usingthe wireless transmitter. In a typical conversation, each person speaksless that 35% of the time, which means that most of the time the reversevoice channel is not being modulated due to voice. With or withoutvoice, the reverse channel is continuously modulated by SAT, which isused by the wireless communications system to monitor channel status.The SAT modulation rate is only about 6 KHz. The voice channels supportin-band messages that are used for hand-off control and for otherreasons, such as for establishing a 3-way call, for answering a secondincoming call while already on a first call, or for responding to an‘audit’ message from the wireless communications system. All of thesemessages, though carried on the voice channel, have characteristicssimilar to the control channel messages. These messages are transmittedinfrequently, and location systems have ignored these messages andfocused on the more prevalent SAT transmissions as the signal ofinterest.

[0344] In view of the above-described difficulties presented by thelimited bandwidth of the FM voice and SAT reverse voice channel signals,an object of the present invention is to provide an improved method bywhich reverse voice channel (RVC) signals may be utilized to locate awireless transmitter, particularly in an emergency situation. Anotherobject of the invention is to provide a location method that allows thelocation system to avoid making location estimates using RVC signals insituations in which it is likely that the measurement will not meetprescribed accuracy and reliability requirements. This saves systemresources and improves the location system's overall efficiency. Theimproved method is based upon two techniques. FIG. 10A is a flowchart ofa first method in accordance with the present invention for measuringlocation using reverse voice channel signals. The method comprises thefollowing steps:

[0345] (i) It is first assumed that a user with a wireless transmitterwishes to be located, or wishes to have his location updated or improvedupon. This may be the case, for example, if the wireless user has dialed“911” and is seeking emergency assistance. It is therefore also assumedthat the user is coherent and in communication with a centrally locateddispatcher.

[0346] (ii) When the dispatcher desires a location update for aparticular wireless transmitter, the dispatcher sends a location updatecommand with the identity of the wireless transmitter to the WirelessLocation System over an application interface.

[0347] (iii) The Wireless Location System responds to the dispatcherwith a confirmation that the Wireless Location System has queried thewireless communications system and has obtained the voice channelassignment for the wireless transmitter.

[0348] (iv) The dispatcher instructs the wireless user to dial a 9 ormore digit number and then the “SEND” button. This sequence may besomething like “123456789” or “911911911”. Two functions happen to thereverse voice channel when the wireless user dial a sequence of at least9 digits and then the “SEND” button. First, especially for an AMPScellular voice channel, the dialing of digits causes the sending of dualtone multi-frequency (DTMF) tones over the voice channel. The modulationindex of DTMF tones is very high and during the sending of each digit inthe DTMF sequence will typically push the bandwidth of the transmittedsignal beyond +/−10 KHz. The second function occurs at the pressing ofthe “SEND” button. Whether or not the wireless user subscribes to 3-waycalling or other special features, the wireless transmitter will send amessage over the voice using a “blank and burst” mode where thetransmitter briefly stops sending the FM voice and SAT, and insteadsends a bursty message modulated in the same manner as the controlchannel (10 Kbits Manchester). If the wireless user dials less than 9digits, the message will be comprised of approximately 544 bits. If thewireless user dials 9 or more digits, the message is comprised ofapproximately 987 bits.

[0349] (v) After notification by the dispatcher, the Wireless LocationSystem monitors the bandwidth of the transmitted signal in the voicechannel. As discussed earlier, when only the SAT is being transmitted,and even if voice and SAT are being transmitted, there may not besufficient bandwidth in the transmitted signal to calculate a highquality location estimate. Therefore, the Wireless Location Systemconserves location processing resources and waits until the transmittedsignal exceeds a predetermined bandwidth. This may be, for example, setsomewhere in the range of 8 KHz to 12 KHz. When the DTMF dialed digitsare sent or when the bursty message is sent, the bandwidth wouldtypically exceed the predetermined bandwidth. In fact, if the wirelesstransmitter does transmit the DTMF tones during dialing, the bandwidthwould be expected to exceed the predetermined bandwidth multiple times.This would provide multiple opportunities to perform a locationestimate. If the DTMF tones are not sent during dialing, the burstymessage is still sent at the time of pressing “SEND”, and the bandwidthwould typically exceed the predetermined threshold.

[0350] (vi) Only when the transmitted bandwidth of the signal exceedsthe predetermined bandwidth, the Wireless Location System initiateslocation processing.

[0351]FIG. 10B is a flowchart of another method in accordance with thepresent invention for measuring location using reverse voice channelsignals. The method comprises the following steps:

[0352] (i) It is first assumed that a user with a wireless transmitterwishes to be located, or wishes to have their location updated orimproved upon. This may be the case, for example, if the wireless userhas dialed “911” and is seeking emergency assistance. It is assumed thatthe user may not wish to dial digits or may not be able to dial anydigits in accordance with the previous method.

[0353] (ii) When the dispatcher desires a location update for aparticular wireless transmitter user, the dispatcher sends a locationupdate command to the Wireless Location System over an applicationinterface with the identity of the wireless transmitter.

[0354] (iii) The Wireless Location System responds to the dispatcherwith a confirmation.

[0355] (iv) The Wireless Location System commands the wirelesscommunications system to make the wireless transmitter transmit bysending an “audit” or similar message to the wireless transmitter. Theaudit message is a mechanism by which the wireless communications systemcan obtain a response from the wireless transmitter without requiring anaction by the end-user and without causing the wireless transmitter toring or otherwise alert. The receipt of an audit message causes thewireless transmitter to respond with an “audit response” message on thevoice channel.

[0356] (v) After notification by the dispatcher, the Wireless LocationSystem monitors the bandwidth of the transmitted signal in the voicechannel. As discussed earlier, when only the SAT is being transmitted,and even if voice and SAT are being transmitted, there may not besufficient bandwidth in the transmitted signal to calculate a highquality location estimate. Therefore, the radio location conserveslocation processing resources and waits until the transmitted signalexceeds a predetermined bandwidth. This may be, for example, setsomewhere in the range of 8 KHz to 12 KHz. When the audit responsemessage is sent, the bandwidth would typically exceed the predeterminedbandwidth.

[0357] (vi) Only when the transmitted bandwidth of the signal exceedsthe predetermined bandwidth, the Wireless Location System initiateslocation processing.

[0358] Estimate Combination Method for Improving Location Accuracy

[0359] The accuracy of the location estimate provided by the WirelessLocation System may be improved by combining multiplestatistically-independent location estimates made while the wirelesstransmitter is maintaining its position. Even when a wirelesstransmitter is perfectly stationary, the physical and RF environmentaround a wireless transmitter is constantly changing. For example,vehicles may change their position or another wireless transmitter whichhad caused a collision during one location estimate may have stoppedtransmitting or changed its position so as to no longer collide duringsubsequent location estimates. The location estimate provided by theWireless Location System will therefore change for each transmission,even if consecutive transmissions are made within a very short period oftime, and each location estimate is statistically independent of theother estimates, particularly with respect to the errors caused by thechanging environment.

[0360] When several consecutive statistically independent locationestimates are made for a wireless transmitter that has not changed itsposition, the location estimates will tend to cluster about the trueposition. The Wireless Location System combines the location estimatesusing a weighted average or other similar mathematical construct todetermine the improved estimate. The use of a weighted average is aidedby the assignment of a quality factor to each independent locationestimate. This quality factor may be based upon, for example, thecorrelation values, confidence interval, or other similar measurementsderived from the location processing for each independent estimate. TheWireless Location System optionally uses several methods to obtainmultiple independent transmissions from the wireless transmitter,including (i) using its interface to the wireless communications systemfor the Make Transmit command; (ii) using multiple consecutive burstsfrom a time slot based air interface protocol, such as TDMA or GSM; or(iii) dividing a voice channel transmission into multiple segments overa period of time and performing location processing independently foreach segment. As the Wireless Location System increases the number ofindependent location estimates being combined into the final locationestimate, it monitors a statistic indicating the quality of the cluster.If the statistic is below a prescribed threshold value, then theWireless Location System assumes that the wireless transmitter ismaintaining its position. If the statistic rises above the prescribedthreshold value, the Wireless Location System assume that the wirelesstransmitter is not maintaining its position and therefore ceases toperform additional location estimates. The statistic indicating thequality of the cluster may be, for example, a standard deviationcalculation or a root mean square (RMS) calculation for the individuallocation estimates being combined together and with respect to thedynamically calculated combined location estimate. When reporting alocation record to a requesting application, the Wireless LocationSystem indicates, using a field in the location record, the number ofindependent location estimate combined together to produce the reportedlocation estimate.

[0361] Another exemplary process for obtaining and combining multiplelocation estimates will now be explained with reference to FIGS.11A-11D. FIGS. 11A, 11B and 11C schematically depict the well-known“origination”, “page response,” and “audit” sequences of a wirelesscommunications system. As shown in FIG. 11A, the origination sequence(initiated by the wireless phone to make a call) may require twotransmissions from the wireless transmitter, an “originate” signal andan “order confirmation” signal. The order confirmation signal is sent inresponse to a voice channel assignment from the wireless communicationssystem (e.g., MSC). Similarly, as shown in FIG. 11B, a page sequence mayinvolve two transmissions from the wireless transmitter. The pagesequence is initiated by the wireless communications system, e.g., whenthe wireless transmitter is called by another phone. After being paged,the wireless transmitter transmits a page response; and then, afterbeing assigned a voice channel, the wireless transmitter transmits anorder confirmation signal. The audit process, in contrast, elicits asingle reverse transmission, an audit response signal. An audit andaudit response sequence has the benefit of not ringing the wirelesstransmitter which is responding.

[0362] The manner in which these sequences may be used to locate a phonewith improved accuracy will now be explained. According to the presentinvention, for example, a stolen phone, or a phone with a stolen serialnumber, is repeatedly pinged with an audit signal, which forces it torespond with multiple audit responses, thus permitting the phone to belocated with greater accuracy. To use the audit sequence, however, theWireless Location System sends the appropriate commands using itsinterface to the wireless communications system, which sends the auditmessage to the wireless transmitter. The Wireless Location System canalso force a call termination (hang up) and then call the wirelesstransmitter back using the standard ANI code. The call can be terminatedeither by verbally instructing the mobile user to disconnect the call,by disconnecting the call at the landline end of the call, or by sendingan artificial over-the-air disconnect message to the base station. Thisover-the-air disconnect message simulates the pressing of the “END”button on a mobile unit. The call-back invokes the above-describedpaging sequence and forces the phone to initiate two transmissions thatcan be utilized to make location estimates.

[0363] Referring now to FIG. 11D, the inventive high accuracy locationmethod will now be summarized. First, an initial location estimate ismade. Next, the above-described audit or “hang up and call back” processis employed to elicit a responsive transmission from the mobile unit,and then a second location estimate is made. Whether the audit or “hangup and call back” process is used will depend on whether the wirelesscommunications system and wireless transmitter have both implemented theaudit functionality. Steps second and third steps are repeated to obtainhowever many independent location estimates are deemed to be necessaryor desirable, and ultimately the multiple statistically-independentlocation estimates are combined in an average, weighted average, orsimilar mathematical construct to obtain an improved estimate. The useof a weighted average is aided by the assignment of a quality factor toeach independent location estimate. This quality factor may be basedupon a correlation percentage, confidence interval, or other similarmeasurements derived from the location calculation process.

[0364] Bandwidth Synthesis Method for Improving Location Accuracy

[0365] The Wireless Location System is further capable of improving theaccuracy of location estimates for wireless transmitters whose bandwidthis relatively narrow using a technique of artificial bandwidthsynthesis. This technique can applied, for example, to thosetransmitters that use the AMPS, NAMPS, TDMA, and GSM air interfaceprotocols and for which there are a large number of individual RFchannels available for use by the wireless transmitter. For exemplarypurposes, the following description shall refer to AMPS-specificdetails; however, the description can be easily altered to apply toother protocols. This method relies on the principle that each wirelesstransmitter is operative to transmit only narrowband signals atfrequencies spanning a predefined wide band of frequencies that is widerthan the bandwidth of the individual narrowband signals transmitted bythe wireless transmitter. This method also relies on the aforementionedinterface between the Wireless Location System and the wirelesscommunications system over which the WLS can command the wirelesscommunications system to make a wireless transmitter handoff or switchto another frequency or RF channel. By issuing a series of commands, theWireless Location System can force the wireless transmitter to switchsequentially and in a controlled manner to a series of RF channels,allowing the WLS effectively to synthesize a wider band received signalfrom the series of narrowband transmitted signals for the purpose oflocation processing.

[0366] In a presently preferred embodiment of the invention, thebandwidth synthesis means includes means for determining a widebandphase versus frequency characteristic of the transmissions from thewireless transmitter. For example, the narrowband signals typically havea bandwidth of approximately 20 KHz and the predefined wide band offrequencies spans approximately 12.5 MHz, which in this example, is thespectrum allocated to each cellular carrier by the FCC. With bandwidthsynthesis, the resolution of the TDOA measurements can be increased toabout 1/12.5 MHz; i.e., the available time resolution is the reciprocalof the effective bandwidth.

[0367] A wireless transmitter, a calibration transmitter (if used),SCS's 10A, 10B and 10C, and a TLP 12 are shown in FIG. 12A. The locationof the calibration transmitter and all three SCS's are accurately knowna priori. Signals, represented by dashed arrows in FIG. 12A, aretransmitted by the wireless transmitter and calibration transmitter, andreceived at SCS's 10A, 10B and 10C, and processed using techniquespreviously described. During the location processing, RF data from oneSCS (e.g. 10B) is cross-correlated (in the time or frequency domain)with the data stream from another SCS (e.g. 10C) separately for eachtransmitter and for each pair of SCS's 10 to generate TDOA estimatesTDOA₂₃ and TDOA₁₃. An intermediate output of the location processing isa set of coefficients representing the complex cross-power as a functionof frequency (e.g., R₂₃).

[0368] For example, if X(f) is the Fourier transform of the signal x(t)received at a first site and Y(f) is the Fourier transform of the signaly(t) received at a second site, then the complex cross-powerR(f)=X(f)Y*(f), wherein Y* is the complex conjugate of Y. The phaseangle of R(f) at any frequency f equals the phase of X(f) minus thephase of Y(f). The phase angle of R(f) may be called the fringe phase.In the absence of noise, interference, and other errors, the fringephase is a perfectly linear function of frequency within a (contiguous)frequency band observed; and slope of the line is minus theinterferometric group delay, or TDOA; the intercept of the line at theband center frequency, equal to the average value of the phase of R(f),is called “the” fringe phase of the observation when reference is beingmade to the whole band. Within a band, the fringe phase may beconsidered to be a function of frequency.

[0369] The coefficients obtained for the calibration transmitter arecombined with those obtained for the wireless transmitter and thecombinations are analyzed to obtain calibrated TDOA measurements TDOA₂₃and TDOA₁₃, respectively. In the calibration process, the fringe phaseof the calibration transmitter is subtracted from the fringe phase ofthe wireless transmitter in order to cancel systematic errors that arecommon to both. Since each original fringe phase is itself thedifference between the phases of signals received at two SCS's 10, thecalibration process is often called double-differencing and thecalibrated result is said to be doubly-differenced. TDOA estimate T−ijis a maximum-likelihood estimate of the time difference of arrival(TDOA), between sites i and j, of the signal transmitted by the wirelesstransmitter, calibrated and also corrected for multipath propagationeffects on the signals. TDOA estimates from different pairs of cellsites are combined to derive the location estimate. It is well knownthat more accurate TDOA estimates can be obtained by observing a widerbandwidth. It is generally not possible to increase the “instantaneous”bandwidth of the signal transmitted by a wireless transmitter, but it ispossible to command a wireless transmitter to switch from one frequencychannel to another so that, in a short time, a wide bandwidth can beobserved.

[0370] In a typical non-wireline cellular system, for example, channels313-333 are control channels and the remaining 395 channels are voicechannels. The center frequency of a wireless transmitter transmitting onvoice RF channel number 1 (RVC 1) is 826.030 MHz and thecenter-to-center frequency spacing of successive channels of 0.030 MHz.The number of voice channels assigned to each cell of a typicalseven-cell frequency-reuse block is about 57 (i.e., 395 divided by 7)and these channels are distributed throughout the 395-channel range,spaced every 7 channels. Note then that each cell site used in an AMPSsystem has channels that span the entire 12.5 MHz band allocated by theFCC. If, for example, we designate cells of each frequency set in are-use pattern as cells “A” through “G”, the channel numbers assigned tothe “A” cell(s) might be 1, 8, 15, 22, . . . , 309; the numbers of thechannels assigned to the “B” cells are determined by adding 1 to the “A”channel numbers; and so on through G.

[0371] The method begins when the wireless transmitter has been assignedto a voice RF channel, and the Wireless Location System has triggeredlocation processing for the transmissions from the wireless transmitter.As part of the location processing, the TDOA estimates TDOA₁₃ and TDOA₂₃combined may have, for example, a standard deviation error of 0.5microsecond. The method combining measurements from different RFchannels exploits the relation between TDOA, fringe phase, and radiofrequency. Denote the “true” value of the group delay or TDOA, i.e., thevalue that would be observed in the absence of noise, multipath, and anyinstrumental error, by τ; similarly, denote the true value of fringephase by φ; and denote the radio frequency by f. The fringe phase φ isrelated to τ and f by:

φ=−fτ+n  (Eq. 1)

[0372] where φ is measured in cycles, f in Hz and τ in seconds; and n isan integer representing the intrinsic integer-cycle ambiguity of adoubly-differenced phase measurement. The value of n is unknown a prioribut is the same for observations at contiguous frequencies, i.e., withinany one frequency channel. The value of n is generally different forobservations at separated frequencies. τ can be estimated fromobservations in a single frequency channel is, in effect, by fitting astraight line to the fringe phase observed as a function of frequencywithin the channel. The slope of the best-fitting line equals minus thedesired estimate τ. In the single-channel case, n is constant and so Eq.1 can be differentiated to obtain:

dφ/df=−τ  (Eq. 2).

[0373] Independent estimates of τ are obtainable by straight-linefitting to the observations of φ vs. f separately for each channel, butwhen two separate (non-contiguous) frequency channels are observed, asingle straight line will not generally fit the observations of φ vs. ffrom both channels because, in general, the integer n has differentvalues for the two channels. However, under certain conditions, it ispossible to determine and remove the difference between these twointeger values and then to fit a single straight line to the entire setof phase data spanning both channels. The slope of this straight linewill be much better determined because it is based on a wider range offrequencies. Under certain conditions, the uncertainty of the slopeestimate is inversely proportional to the frequency span.

[0374] In this example, suppose that the wireless transmitter has beenassigned to voice RF channel 1. The radio frequency difference betweenchannels 1 and 416 is so great that initially the difference between theintegers n₁ and n₄₁₆ corresponding to these channels cannot bedetermined. However, from the observations in either or both channelstaken separately, an initial TDOA estimate τ₀ can be derived. Now theWireless Location System commands the wireless communications system tomake the wireless transmitter to switch from channel 1 to channel 8. Thewireless transmitter's signal is received in channel 8 and processed toupdate or refine the estimate τ₀. From τ₀, the “theoretical”fringe-phase φ₀ as a function of frequency can be computed, equal to(−fτ₀). The difference between the actually observed phase φ and thetheoretical function φ₀ can be computed, wherein the actually observedphase equals the true phase within a very small fraction, typically{fraction (1/50)}th, of a cycle:

φ−φ₀ =−f(τ−τ₀)+n ₁ or n ₈, depending on the channel  (Eq. 3)

[0375] or

Δφ=−Δfτ−n ₁ or n ₈, depending on the channel  (Eq. 4)

[0376] where Δφ≡φ−φ₀ and Δτ≡τ−τ₀. Equation (4) is graphed in FIG. 12B,depicting the difference, Δφ, between the observed fringe phase φ andthe value φ₀ computed from the initial TDOA estimate τ₀, versusfrequency f for channels 1 and 8.

[0377] For the 20 KHz-wide band of frequencies corresponding to channel1, a graph of Δφ vs. f is typically a horizontal straight line. For the20 KHz-wide band of frequencies corresponding to channel 8, the graph ofΔφ vs. f is also horizontal straight line. The slopes of these linesegments are generally nearly zero because the quantity (fΔτ) usuallydoes not vary by a significant fraction of a cycle within 20 KHz,because Δτ is minus the error of the estimate τ₀. The magnitude of thiserror typically will not exceed 1.5 microseconds (3 times the standarddeviation of 0.5 microseconds in this example), and the product of 1.5microseconds and 20 KHz is under 4% of a cycle. In FIG. 12B, the graphof Δφ for channel 1 is displaced vertically from the graph of Δφ forchannel 8 by a relatively large amount because the difference between n₁and n₈ can be arbitrarily large. This vertical displacement, ordifference between the average values of Δφ for channels 1 and 8, will(with extremely high probability) be within ±0.3 cycle of the true valueof the difference, n₁ and n₈, because the product of the maximum likelymagnitude of Δτ (1.5 microseconds) and the spacing of channels 1 and 8(210 KHz) is 0.315 cycle. In other words, the difference n₁−n₈ is equalto the difference between the average values of Δφ for channels 1 and 8,rounded to the nearest integer. After the integer difference n₁−n₈ isdetermined by this rounding procedure, the integer Δφ is added forchannel 8 or subtracted from Δφ for channel 1. The difference betweenthe average values of Δφ for channels 1 and 8 is generally equal to theerror in the initial TDOA estimate, τ₀ , times 210 KHz. The differencebetween the average values of Δφ for channels 1 and 8 is divided by 210KHz and the result is added to τ₀ to obtain an estimate of τ, the truevalue of the TDOA; this new estimate can be significantly more accuratethan τ₀.

[0378] This frequency-stepping and TDOA-refining method can be extendedto more widely spaced channels to obtain yet more accurate results. Ifτ₁ is used to represent the refined result obtained from channels 1 and8, τ₀ can be replaced by τ₁ in the just-described method; and theWireless Location System can command the wireless communications systemto make the wireless transmitter switch, e.g., from channel 8 to channel36; then τ₁ can be used to determine the integer difference n₈−n₃₆ and aTDOA estimate can be obtained based on the 1.05 MHz frequency spanbetween channels 1 and 36. The estimated can be labeled τ₂; and thewireless transmitter switched, e.g., from channel 36 to 112, and so on.In principle, the full range of frequencies allocated to the cellularcarrier can be spanned. The channel numbers (1, 8, 36, 112) used in thisexample are, of course, arbitrary. The general principle is that anestimate of the TDOA based on a small frequency span (starting with asingle channel) is used to resolve the integer ambiguity of the fringephase difference between more widely separated frequencies. The latterfrequency separation should not be too large; it is limited by theuncertainty of the prior estimate of TDOA. In general, the worst-caseerror in the prior estimate multiplied by the frequency difference maynot exceed 0.5 cycle.

[0379] If the very smallest (e.g., 210 KHz) frequency gap between themost closely spaced channels allocated to a particular cell cannot bebridged because the worst-case uncertainty of the single-channel TDOAestimate exceeds 2.38 microseconds (equal to 0.5 cycle divided by 0.210MHz), the Wireless Location System commands the wireless communicationssystem to force the wireless transmitter hand-off from one cell site toanother (e.g. from one frequency group to another), such that thefrequency step is smaller. There is a possibility of misidentifying theinteger difference between the phase differences (Δφ's) for twochannels, e.g., because the wireless transmitter moved during thehandoff from one channel to the other. Therefore, as a check, theWireless Location System may reverse each handoff (e.g., after switchingfrom channel 1 to channel 8, switch from channel 8 back to channel 1)and confirm that the integer-cycle difference determined has preciselythe same magnitude and the opposite sign as for the “forward” hand-off.A significantly nonzero velocity estimate from the single-channel FDOAobservations can be used to extrapolate across the time intervalinvolved in a channel change. Ordinarily this time interval can be heldto a small fraction of 1 second. The FDOA estimation error multiplied bythe time interval between channels must be small in comparison with 0.5cycle. The Wireless Location System preferably employs a variety ofredundancies and checks against integer-misidentification.

[0380] Directed Retry for 911

[0381] Another inventive aspect of the Wireless Location System relatesto a “directed retry” method for use in connection with a dual-modewireless communications system supporting at least a first modulationmethod and a second modulation method. In such a situation, the firstand second modulation methods are assumed to be used on different RFchannels (i.e. channels for the wireless communications systemsupporting a WLS and the PCS system, respectively). It is also assumedthat the wireless transmitter to be located is capable of supportingboth modulation methods, i.e. is capable of dialing “911” on thewireless communications system having Wireless Location System support.

[0382] For example, the directed retry method could be used in a systemin which there are an insufficient number of base stations to support aWireless Location System, but which is operating in a region served by aWireless Location System associated with another wireless communicationssystem. The “first” wireless communications system could be a cellulartelephone system and the “second” wireless communications system couldbe a PCS system operating within the same territory as the first system.According to the invention, when the mobile transmitter is currentlyusing the second (PCS) modulation method and attempts to originate acall to 911, the mobile transmitter is caused to switch automatically tothe first modulation method, and then to originate the call to 911 usingthe first modulation method on one of the set of RF channels prescribedfor use by the first wireless communications system. In this manner,location services can be provided to customers of a PCS or like systemthat does is not served by its own Wireless Location System.

[0383] Modified Transmission Method for Improving Accuracy for E9-1-1Calls

[0384] The accuracy of the location estimate of the Wireless LocationSystem is dependent, in part, upon both the transmitted power of thewireless transmitter and the length in time of the transmission from thewireless transmitter. In general, higher power transmissions andtransmissions of greater transmission length can be located with betteraccuracy by the Wireless Location System than lower power and shortertransmissions. These transmission characteristics of higher power andlonger lengths are not attractive for wireless communications systems,however. Wireless communications systems generally limit the transmitpower and transmission length of wireless transmitters in order tominimize interference within the communications system and to maximizethe potential capacity of the system. The following method meets theconflicting needs of both systems by enabling the wirelesscommunications system to minimize transmit power and length whileenabling improved location accuracy for certain types of calls, such aswireless 9-1-1 calls.

[0385] The transmitted power and length of the transmission aretypically controlled by the wireless communications system. That is, awireless transmitter will receive parameters from the forward controlchannels of a base station in a wireless communications system, and theparameters will define power and transmission length for all phones andall wireless transmissions to that base station. By way of example, inan IS-136 (TDMA) type of system, the base station may set a parameterknown as DMAC to 4, which defines the output power of a wirelesstransmitter's control channel transmission to be 8 dB less than fullportable power, or approximately 100 m Watts. Further, the base stationmay set origination transmissions to have a length of 2 bursts, or 13.4milliseconds, by minimizing the number of fields included in thetransmission. For improved accuracy, the Wireless Location System wouldprefer transmissions of greater power, 600 mWatts for example, andlengths of 3 or more bursts, which can be achieved by enabling fieldssuch as “Authentication”, “Serial Number”, or “Mobile Assisted ChannelAllocation Report”. AMPS, CDMA, GSM, and iDEN systems similarly haveparameters controlling transmissions within those networks.

[0386] The following method can be used to improve the accuracy ofspecific types of calls from wireless transmitters, such as calls to“9-1-1”. This might be important because, for example, particular typesof calls might have greater accuracy requirements than other types ofcalls. Wireless calls to 9-1-1, for example, have very specific accuracyrequirements defined by the Federal Communications Commission that maynot apply to other types of calls. Therefore, this method isparticularly inventive for wireless calls to 9-1-1 because in the UnitedStates, the FCC has mandated that “9-1-1” is the only number to callfrom wireless phones for emergencies. This mandated dialing sequenceprovides a consistent dialing sequence to use as a trigger for invokingthis method for emergency calls. Previously, various states and citieshad posted a wide variety of emergency numbers along highways.

[0387] There are two parts to this method to improve accuracy (i)processing logic within the wireless transmitter that detects one ormore trigger events and causes a separate set of transmission parametersto be used, and (ii) processing logic with the Wireless Location Systemthat detects the trigger event and processes the transmission using thedifferent set of transmission parameters.

[0388] Within the wireless transmitter, the following steps areperformed:

[0389] a wireless transmitter listens to the forward control channels ofa wireless communications system and receives the “normal” transmissionparameters broadcast for use by all wireless transmitters;

[0390] the user of a wireless transmitter initiates a call on thewireless transmitter by dialing a sequence of digits and pressing “SEND”or “YES”;

[0391] the processor within the wireless transmitter compares the dialedsequence of digits with one or more trigger events stored within thewireless transmitter (in this example, the trigger event may be “9-1-1”and/or variations such as “*9-1-1” or “#9-1-1”);

[0392] if the dialed sequence of digits does not match the triggerevent, then the wireless transmitter uses the normal transmissionparameters in making the call; and

[0393] if the dialed sequence of digits matches the trigger event, thenthe wireless transmitter uses a modified transmission sequence.

[0394] The modified transmission sequence consists of one or more of thefollowing steps:

[0395] the wireless transmitter first examines the normal parametersbroadcast on the forward channels by the base station to determine thenormal power setting and normal fields to be included in thetransmission;

[0396] the wireless transmitter may increase its transmitted power by apredetermined amount over the power level setting in the normalparameters, up to the maximum power setting;

[0397] the wireless transmitter may increase its transmitted power tothe maximum power setting;

[0398] the wireless transmitter may transmit an additional predeterminednumber of access probes (in the case of certain air interfaces such asCDMA) even after the base station has acknowledged receipt of the accessprobes to the wireless transmitter;

[0399] the wireless transmitter may include additional fields, such as“Authentication”, “Serial Number”, or “Mobile Assisted ChannelAllocation Report” fields, in the transmitted message even if thesefields are not requested in the normal parameters broadcast on theforward channels by the base station;

[0400] the wireless transmitter may follow a transmitted message withone or more repeated registration messages, where each registrationmessage may be of the normal length determined from the transmissionparameters broadcast on the forward channels by the base station, or maybe modified to a longer length by including additional fields, such as“Authentication”, “Serial Number”, “Mobile Assisted Channel AllocationReport”, or “Capability Report” fields; or

[0401] the wireless transmitter may follow a transmitted messagetransmitted on a first one of a plurality of channels with one or morerepeated registration messages transmitted on another second one of aplurality of channels, where each registration message may be of thenormal length determined from the transmission parameters broadcast onthe forward channels by the base station, or may be modified to a longerlength by including additional fields, such as “Authentication”, “SerialNumber”, “Mobile Assisted Channel Allocation Report”, or “CapabilityReport” fields.

[0402] In one of the steps in modified transmission sequence, thewireless transmitter may follow a transmitted message transmitted on afirst one of a plurality of channels with one or more repeatedregistration messages transmitted on another second one of a pluralityof channels. The purpose of this step is to provide the WirelessLocation System with transmissions of both longer length and ondifferent frequencies. By observing transmissions at differentfrequencies, the Wireless Location System can potentially improve itslocation processing by better mitigating multipath and reducing noisedue to interference. In selecting another second one of a plurality ofchannels, the wireless transmitter may modify its channel selectionprocess by:

[0403] selecting another second channel in use by a second base stationwithin listening range of the wireless transmitter and for which thewireless transmitter can receive the forward control channel broadcastby that second base station (the second base station may be the same asthe first base station, or another sector of the first base station, oran entirely separate base station); or

[0404] selecting another second channel for which the wirelesstransmitter can detect no forward control channel activity by any basestation (in this case, the wireless transmitter will transmit one ormore registration messages without expecting any acknowledgement fromthe wireless communications system).

[0405] In some wireless communication systems, transmitted messages mayfollow one of several message encryption schemes defined in protocolssuch as TDMA, CDMA, or GSM. These encryption schemes are designed, inpart, to prevent systems other than base stations from correctlyinterpreting the content of the messages transmitted by wirelesstransmitters. As a further step in this method, and in addition to thesteps detailed above, the wireless transmitter may optionally deactivateencryption when a trigger event occurs and for all messages transmittedas part of the modified transmission sequence.

[0406] By using a trigger event as the only time in which the wirelesstransmitter modifies its transmission from the normal parametersbroadcast by the base station, the wireless transmitter greater reducesthe number of times in which the modified transmissions are used andtherefore greater reduces the probability of increased interference tothe wireless communications system caused by not using normalparameters. For example, this can be a significant advantage toincreasing the location accuracy of 9-1-1 emergency calls, withoutmeasurable degradation to the remainder of a wireless network's callprocessing. While wireless 9-1-1 calls have great importance, the actualdensity of wireless 9-1-1 calls is very low when compared to all othercalls in a wireless network. Across the U.S., there are an average ofonly 1.5 wireless 9-1-1 calls per cell site per day. Therefore, there islikely to be a very low incidence of interference to wireless networkscaused by increased transmission power or transmission length during9-1-1 calls. Even if an interference incident were caused by a phoneusing the methods of this invention, normal call processing within allexisting air interface protocols provides for back-off and re-attempt bythe phone receiving in the interference. Therefore, this method shouldnever cause non-emergency call attempts to fail. While the above methodhas been described for calls using the dialed digits “9-1-1” andvariations, the method can be applied to other types of triggered eventsas well. Finally, the trigger events may be permanently stored in thewireless transmitter, programmed by the user into the wirelesstransmitter, or broadcast by the wireless communications system forreceipt by all wireless transmitters. Further, the actions to be takenduring the modified transmission sequence may be permanently stored inthe wireless transmitter, programmed by the user into the wirelesstransmitter, or broadcast by the wireless communications system forreceipt by all wireless transmitters.

[0407] The Wireless Location System is capable of independentlydemodulating transmissions on multiple channels, and can thereforedetect and process for location purposes the entirety of all messagessent from the wireless transmitter, including all of the modifiedtransmission sequences described above. In most, if not all, cases thebase station will ignore additional fields sent in a message by thewireless transmitter. Further, the additional registration messages willalso have no effect on call processing by the base station. Therefore,the additional actions described above will have the primary effect ofaiding the Wireless Location System in improving the accuracy of thelocation estimate without degrading the performance of the wirelesscommunications system.

[0408] Conclusion

[0409] The true scope the present invention is not limited to thepresently preferred embodiments disclosed herein. For example, theforegoing disclosure of a presently preferred embodiment of a WirelessLocation System uses explanatory terms, such as Signal Collection System(SCS), TDOA Location Processor (TLP), Applications Processor (AP), andthe like, which should not be construed so as to limit the scope ofprotection of the following claims, or to otherwise imply that theinventive aspects of the Wireless Location System are limited to theparticular methods and apparatus disclosed. Moreover, as will beunderstood by those skilled in the art, many of the inventive aspectsdisclosed herein may be applied in location systems that are not basedon TDOA techniques. For example, the processes by which the WirelessLocation System uses the Tasking List, etc. can be applied to non-TDOAsystems. In such non-TDOA systems, the TLP's described above would notbe required to perform TDOA calculations. Similarly, the invention isnot limited to systems employing SCS's constructed as described above,nor to systems employing AP's meeting all of the particulars describedabove. The SCS's, TLP's and AP's are, in essence, programmable datacollection and processing devices that could take a variety of formswithout departing from the inventive concepts disclosed herein. Giventhe rapidly declining cost of digital signal processing and otherprocessing fuctions, it is easily possible, for example, to transfer theprocessing for a particular function from one of the functional elements(such as the TLP) described herein to another functional element (suchas the SCS or AP) without changing the inventive operation of thesystem. In many cases, the place of implementation (i.e. the functionalelement) described herein is merely a designer's preference and not ahard requirement. Accordingly, except as they may be expressly solimited, the scope of protection of the following claims is not intendedto be limited to the specific embodiments described above.

What is claimed is:
 1. A method for use in a Wireless Location System(WLS) in locating a mobile wireless transmitter, comprising the stepsof: a) a wireless transmitter receives normal transmission parametersfrom a base station; b) the user of the wireless transmitter initiates acall on the wireless transmitter by dialing a sequence of digits andpressing “SEND” or “YES”; c) a processor within the wireless transmittercompares the dialed sequence of digits with one or more trigger eventsstored within the wireless transmitter; d) if the dialed sequence ofdigits does not match the trigger event, then the wireless transmitteruses the normal transmission parameters in making the call; and e) ifthe dialed sequence of digits matches the trigger event, then thewireless transmitter uses a modified transmission sequence.
 2. A methodas recited in claim 1, wherein the modified transmission sequencecomprises the wireless transmitter increasing its transmitted power by apredetermined amount over the power level setting in the normalparameters, up to a maximum power setting.
 3. A method as recited inclaim 1, wherein the modified transmission sequence comprises thewireless transmitter increasing its transmitted power to the maximumpower setting.
 4. A method as recited in claim 1, wherein the modifiedtransmission sequence comprises the wireless transmitter transmitting anadditional predetermined number of access probes even after the basestation has acknowledged receipt of the access probes to the wirelesstransmitter.
 5. A method as recited in claim 1, wherein the modifiedtransmission sequence comprises the wireless transmitter includingadditional fields in the transmitted message even if these fields arenot requested in the normal parameters broadcast on forward channels bythe base station.
 6. A method as recited in claim 1, wherein themodified transmission sequence comprises the wireless transmitterfollowing a transmitted message with one or more repeated registrationmessages.
 7. A method as recited in claim 1, wherein the modifiedtransmission sequence comprises the wireless transmitter following atransmitted message transmitted on a first one of a plurality ofchannels with one or more repeated registration messages transmitted onanother second one of a plurality of channels.
 8. A method as recited inclaim 5, wherein the additional field is an Authentication field.
 9. Amethod as recited in claim 5, wherein the additional field is a SerialNumber field.
 10. A method as recited in claim 5, wherein the additionalfield is a Mobile Assisted Channel Allocation Report field.
 11. A methodas recited in claim 6, wherein the one or more repeated registrationmessages may be of the normal length determined from the transmissionparameters broadcast on the forward channels by the base station.
 12. Amethod as recited in claim 6, wherein the one or more repeatedregistration messages may be modified to a longer length by includingadditional fields.
 13. A method as recited in claim 7, wherein the oneor more repeated registration messages may be of the normal lengthdetermined from the transmission parameters broadcast on the forwardchannels by the base station.
 14. A method as recited in claim 7,wherein the one or more repeated registration messages may be modifiedto a longer length by including additional fields.
 15. A method asrecited in claim 12 or 14, wherein the additional field is anAuthentication field.
 16. A method as recited in claim 12 or 14, whereinthe additional field is a Serial Number field.
 17. A method as recitedin claim 12 or 14, wherein the additional field is a Mobile AssistedChannel Allocation Report field.
 18. A method as recited in claim 12 or14, wherein the additional field is a Capability Report field.
 19. Amethod as recited in claim 7, wherein the second one of a plurality ofchannels is selected to be one in use by a second base station withinlistening range of the wireless transmitter and for which the wirelesstransmitter can receive the forward control channel broadcast by thatsecond base station.
 20. A method as recited in claim 7, wherein thesecond one of a plurality of channels is selected to be one for whichthe wireless transmitter can detect no forward control channel activityby any base station.
 21. A method as recited in claim 19, wherein thesecond base station may be the same as the first base station, anothersector of the first base station, or an entirely separate base station.22. A method as recited in claim 20, wherein the wireless transmitterwill transmit the one or more registration messages without expectingany acknowledgement from the wireless communications system.
 23. Amethod as recited in claim 1, wherein the trigger events are permanentlystored in the wireless transmitter.
 24. A method as recited in claim 1,wherein the trigger events are programmed by the user into the wirelesstransmitter.
 25. A method as recited in claim 1, wherein the triggerevents are broadcast by the wireless communications system for receiptby a plurality of wireless transmitters.
 26. A method as recited inclaim 1, wherein the actions to be taken during the modifiedtransmission sequence may be permanently stored in the wirelesstransmitter.
 27. A method as recited in claim 1, wherein the actions tobe taken during the modified transmission sequence are programmed by theuser into the wireless transmitter.
 28. A method as recited in claim 1,wherein the actions to be taken during the modified transmissionsequence are broadcast by the wireless communications system for receiptby all wireless transmitters.
 29. A method as recited in claim 1,wherein the trigger event includes the dialed digits “9-1-1” and/orvariations such as “*9-1-1” or “#9-1-1”.
 30. A method as recited inclaim 1, wherein the wireless transmitter deactivates encryption when atrigger event occurs.
 31. A method as recited in claim 1, wherein thewireless transmitter deactivates encryption for all messages transmittedas part of the modified transmission sequence.
 32. A wirelesstransmitter for use in a wireless communications system, capable ofperforming the following functions: a) receiving normal transmissionparameters from a base station; b) initiating a call when a user of thewireless transmitter dials a sequence of digits and presses “SEND” or“YES”; c) comparing the dialed sequence of digits with one or moretrigger events stored within the wireless transmitter; d) using thenormal transmission parameters in making the call if the dialed sequenceof digits do not match the trigger event; and e) using a modifiedtransmission sequence if the dialed sequence of digits matches thetrigger event.
 33. A wireless transmitter as recited in claim 32,wherein the modified transmission sequence comprises the wirelesstransmitter increasing its transmitted power by a predetermined amountover the power level setting in the normal parameters, up to a maximumpower setting.
 34. A wireless transmitter as recited in claim 32,wherein the modified transmission sequence comprises the wirelesstransmitter increasing its transmitted power to the maximum powersetting.
 35. A wireless transmitter as recited in claim 32, wherein themodified transmission sequence comprises the wireless transmittertransmitting an additional predetermined number of access probes evenafter the base station has acknowledged receipt of the access probes tothe wireless transmitter.
 36. A wireless transmitter as recited in claim32, wherein the modified transmission sequence comprises the wirelesstransmitter including additional fields in the transmitted message evenif these fields are not requested in the normal parameters broadcast onforward channels by the base station.
 37. A wireless transmitter asrecited in claim 32, wherein the modified transmission sequencecomprises the wireless transmitter following a transmitted message withone or more repeated registration messages.
 38. A wireless transmitteras recited in claim 32, wherein the modified transmission sequencecomprises the wireless transmitter following a transmitted messagetransmitted on a first one of a plurality of channels with one or morerepeated registration messages transmitted on another second one of aplurality of channels.
 39. A wireless transmitter as recited in claim36, wherein the additional field is an Authentication field.
 40. Awireless transmitter as recited in claim 36, wherein the additionalfield is a Serial Number field.
 41. A wireless transmitter as recited inclaim 36, wherein the additional field is a Mobile Assisted ChannelAllocation Report field.
 42. A wireless transmitter as recited in claim36, wherein the one or more repeated registration messages may be of thenormal length determined from the transmission parameters broadcast onthe forward channels by the base station.
 43. A wireless transmitter asrecited in claim 37, wherein the one or more repeated registrationmessages may be modified to a longer length by including additionalfields.
 44. A wireless transmitter as recited in claim 38, wherein theone or more repeated registration messages may be of the normal lengthdetermined from the transmission parameters broadcast on the forwardchannels by the base station.
 45. A wireless transmitter as recited inclaim 38, wherein the one or more repeated registration messages may bemodified to a longer length by including additional fields.
 46. Awireless transmitter as recited in claim 43 or 45, wherein theadditional field is an Authentication field.
 47. A wireless transmitteras recited in claim 43 or 45, wherein the additional field is a SerialNumber field.
 48. A wireless transmitter as recited in claim 43 or 45,wherein the additional field is a Mobile Assisted Channel AllocationReport field.
 49. A wireless transmitter as recited in claim 43 or 45,wherein the additional field is a Capability Report field.
 50. Awireless transmitter as recited in claim 38, wherein the second one of aplurality of channels is selected to be one in use by a second basestation within listening range of the wireless transmitter and for whichthe wireless transmitter can receive the forward control channelbroadcast by that second base station.
 51. A wireless transmitter asrecited in claim 38, wherein the second one of a plurality of channelsis selected to be one for which the wireless transmitter can detect noforward control channel activity by any base station.
 52. A wirelesstransmitter as recited in claim 50, wherein the second base station maybe the same as the first base station, another sector of the first basestation, or an entirely separate base station.
 53. A wirelesstransmitter as recited in claim 51, wherein the wireless transmitterwill transmit the one or more registration messages without expectingany acknowledgement from the wireless communications system.
 54. Awireless transmitter as recited in claim 32, wherein the trigger eventsare permanently stored in the wireless transmitter.
 55. A wirelesstransmitter as recited in claim 32, wherein the trigger events areprogrammed by the user into the wireless transmitter.
 56. A wirelesstransmitter as recited in claim 32, wherein the trigger events arebroadcast by the wireless communications system for receipt by aplurality of wireless transmitters.
 57. A wireless transmitter asrecited in claim 32, wherein the actions to be taken during the modifiedtransmission sequence may be permanently stored in the wirelesstransmitter.
 58. A wireless transmitter as recited in claim 32, whereinthe actions to be taken during the modified transmission sequence areprogrammed by the user into the wireless transmitter.
 59. A wirelesstransmitter as recited in claim 32, wherein the actions to be takenduring the modified transmission sequence are broadcast by the wirelesscommunications system for receipt by all wireless transmitters.
 60. Awireless transmitter as recited in claim 32, wherein the trigger eventincludes the dialed digits “9-1-1” and/or variations such as “*9-1-1” or“#9-1-1”.
 61. A Wireless Location System capable of locating a wirelesstransmitter using a modified transmission sequence, wherein the modifiedtransmission sequence comprises a message sent from the wirelesstransmitter using transmission parameters different from the normaltransmission parameters broadcast on the forward control channel by thebase stations in a wireless communications system.